Assessment and control of odour nuisance from waste water treatment: code of practice

Part 1 Code of Practice on Assessment of Odour Nuisance from Waste Water Treatment Works

3. AN OVERVIEW OF THE PROBLEM

Waste water is produced as a by-product of human existence and numerous industrial processes. Although primarily water, waste water contains various other biological and chemical materials which, if released in an uncontrolled manner to the environment are capable of causing pollution. The production, transmission and treatment of waste water can result in the generation of odour.

There are many different means of preventing, controlling or reducing odours to minimise the impact of odour in the locality of WWTW and avoid creation of odour nuisance. This CoP defines a clear pathway for the development of an action plan for the resolution of odour nuisance. This plan has a series of steps starting with the receipt of a complaint, odour and source assessment and ends up with the measures to prevent odour, and (where that is not practicable) to contain odours and minimise odour emissions. This plan of action should allow all stakeholders to see that the choice of control measures proposed for a specific site has been arrived at in way that is both technically justifiable and takes into account the balance of benefits and costs. This CoP also covers matters to be taken into consideration in the design of future or upgraded works.

Prior to 1996, treatment works were mostly operated by the nine mainland regions and the three island areas of local government. The creation of the three water authorities (East of Scotland Water Authority, North of Scotland Water Authority and West of Scotland Water Authority) in 1996 and the formation of Scottish Water in April 2002 effectively moved WWTW out of local government management.

Scottish Water is responsible for treatment of all municipal waste water in Scotland and serves around 5 million customers. Over 90% of the population of Scotland is connected to a mains sewerage system. In addition, Scottish Water has 20 PPP waste water treatment works that treat approximately 45% of the total waste water produced in Scotland. These are generally the larger WWTW and are governed by 9 PPP contracts.

The incidence of complaints of odour nuisance from WWTW has been steadily increasing over the last two decades. There is little evidence of changes in the raw sewage itself to explain this, but there are a number of other reasons why this might have occurred.

1. The awareness of the public and expectation of a better environment have increased, as has the belief that complaint can lead to action.

2. Housing and other developments have significantly encroached on the land around WWTW increasing the number of people likely to be impacted by odour from WWTW.

3. Implementation of the Urban Waste water Treatment ( UWWT) Directive during the 1990s has been a key environmental driver in requiring improvement of waste water treatment plant and discharges. The Directive primarily requires improvements in the discharge quality from WWTW and has resulted in many works installing additional treatment stages in the process. This has resulted in a substantial investment in the construction of new and upgraded treatment works to meet the water quality requirements of this Directive, many of these works being in odour-sensitive locations. There has also been an increased requirement to pump sewage for considerable distances leading to poor aeration that can result in odour. There are also a number of other legislative drivers that may potentially result in higher levels of treatment than required for the UWWT Directive.

4. Implementation of the UWWT Directive also prohibits the disposal of sludge at sea thereby requiring storage and treatment of sludge before final land disposal.

4. LEGAL FRAMEWORK

4.1 Legal Framework

The regulation of odour emissions from WWTW relies upon the Statutory Nuisance controls detailed in Part III of the Environmental Protection Act 1990 that are enforced by local authorities. These controls require that operators of WWTW do not cause a Statutory Nuisance due to emissions of odours.

Section 80 of the Environmental Protection Act 1990 provides that where a local authority is satisfied that a Statutory Nuisance exists, or is likely to occur or recur, they shall serve a notice (an Abatement Notice). This notice can require the execution of such works and other steps, necessary to abate the nuisance or prohibit its occurrence or recurrence.

Further guidance is included in this Part of the CoP on the investigation and assessment of odour nuisance.

The person on whom an Abatement Notice is served has the right of appeal to the sheriff under the provisions of the Statutory Nuisance (Appeals) (Scotland) Regulations 1996. The grounds for appeal include:-

• that the Notice is not justified (no nuisance exists)
• that the authority have refused to accept alternate means of compliance to those specified in the Notice
• that the time limit specified for compliance is insufficient
• where the Statutory Nuisance is one falling within section 79 (1)(d) (which includes smell which is prejudicial to health or a nuisance) and it is arising from industrial, trade or business premises, that the 'best practicable means' (bpm) have been used to prevent or counteract the effects of the nuisance.

In the case of an appeal against a notice that requires significant expenditure, the notice is usually suspended until the appeal it determined. On hearing the appeal, the Court can quash or vary the notice or dismiss the appeal.

A person who fails to comply with the requirements of an Abatement Notice is guilty of an offence and liable to a fine on summary conviction. In the case of Statutory Nuisance due to odour from industrial, trade or business premises, Section 80 (7) of the Act provides a defence for the operator to demonstrate that the 'best practicable means' (bpm) have been used to prevent or counteract the effects of the nuisance.

In circumstances where a local authority is of the opinion that service of an abatement notice under section 80(4) would afford an inadequate remedy, section 81(5) of the Environmental Protection Act 1990 allows the local authority taking proceedings in the Sheriff Court or the Court of Session to seek an interdict.

Finally, the Environmental Protection Act 1990 provides for actions to be taken by members of the public. Section 82 (1) of the Environmental Protection Act 1990 allows any person on the grounds that he is aggrieved by the existence of a Statutory Nuisance to seek an order from the Sheriff to abate the nuisance and prohibit the recurrence of the nuisance.

4.2 Best Practicable Means (bpm)

As outlined in Section 4.1 above, in the case of Statutory Nuisance due to odour from a WWTW, it is a defence for the operator to demonstrate that the 'best practicable means' (bpm) have been used to prevent or counteract the effects of the nuisance. The term best practicable means (bpm) is defined in section 79(9) of the Environmental Protection Act 1990 as:-

'the term 'best practicable means' is to be interpreted by reference to the following provisions-

a) "practicable" means reasonably practicable having regard among other things to local conditions and circumstances, to the current state of technical knowledge and to the financial implications;

b) the means to be employed include the design, installation, maintenance and manner and periods of operation of plant and machinery, and the design, construction and maintenance of buildings and structures;

c) the test is to apply only so far as compatible with any duty imposed by law;

d) the test is to apply only so far as compatible with safety and safe working conditions, and with the exigencies of any emergency or unforeseeable circumstances;'

The key issue when determining bpm usually relates to the interpretation of 'practicable'. It should be noted that the definition of 'practicable' is not exhaustive as the list details issues that 'among other things' should be taken into account. The definition includes cost consideration but clearly cost is not necessarily the decisive factor. It is finally a matter for the Courts to determine whether in a particular instance the controls adopted are reasonable or the costs are excessive taking account of local conditions and characteristics of the odour nuisance. Finally, it is important to note that it is for the person relying on the defence (that is the WWTW operator) to establish that bpm has been used.

The procedures and controls outlined in this CoP (particularly in Part 2) establish a basis against which the term 'best practicable means' (bpm) as a defence against Statutory Nuisance can be compared.

4.3 WWTW Regulated under other Statutory Controls

This CoP is intended to apply to WWTW where the Environmental Protection Act 1990 is the main or only source of regulation for odour emissions. For example, a relatively small number of WWTW fall under the Integrated Pollution Prevention and Control ( IPPC) regime and are regulated by the Scottish Environment Protection Agency ( SEPA) under the Pollution Prevention and Control ( PPC) Regulations.

The PPC Regulations require that certain operations for the treatment of waste are subject to the IPPC regime under SEPA regulation. The definition of installations subject to these controls is included in the Pollution Prevention and Control (Scotland) Regulations 2000 as amended and outlined below:-

(a) The disposal of hazardous waste (other than by incineration or landfill) in a facility with a capacity of more than 10 tonnes per day.

(b) The disposal of waste oils (other than by incineration or landfill) in a facility with a capacity of more than 10 tonnes per day.

(c) Disposal of non-hazardous waste in a facility with a capacity of more than 50 tonnes per day by -

(i) biological treatment or

(ii) physico-chemical treatment.

In addition, under the provisions of the Waste Management Licensing Amendment (Scotland) Regulations 2004 ( SI 2004 No 275), a waste management licence under Part II of the Environmental Protection Act 1990 is required from SEPA in the following cases:-

a) the treatment of screenings, sludges and septic tank sludge arising within the sewage treatment works at the works where the total quantity in any 12-month period exceeds 10,000 cubic metres

b) the importation to a sewage treatment works for recovery of screenings, sludges and septic tank sludge where the total quantity in any 12-month period exceeds 100,000 cubic metres.

Subject to the guidance in Section 1.3, this CoP will not be applied to those sites and separate guidance on the applicable standards for these processes will be produced in due course by SEPA.

4.4 What this Code applies to

This CoP applies to odour nuisance from WWTW themselves, rather than to the wider sewerage network. For the purpose of this CoP, WWTW with a capacity of less than 500pe are not expected to be major odour sources and hence the requirements for the preparation and implementation of an Odour Management Plan of Part 2 of this CoP should only apply to such works where odour nuisance exists. The other requirements of Part 2 of this CoP should apply in all cases.

4.5 New and Existing Works

This CoP applies to both existing WWTW and new works, as well as to existing works where substantial change is planned. However, it is expected that for new works or substantial changes to existing works operators will want to assess the potential for odour nuisance at the design stage. Consideration of odour control options at the design stage of a project will allow a strategic decision to be made on the measures necessary to avoid the creation of odour nuisance. The incorporation of control measures during the build of new or upgraded plant will limit the need for costly retrofitting of controls at a later date and hence suitable odour controls should be included at the outset.

It is expected that some of the controls in Part 2 (such as the Odour Management Plan) may not be currently in place but all works should meet the minimum standards of this CoP by 1 May 2006.

4.6 Planning Controls and Nuisance

Development of new WWTW, and modifications to existing sites, require planning permission. The Scottish Executive's planning policy for WWTW is included in National Planning Policy Guideline ( NPPG) 10- Planning and Waste Management. Further advice is set out in Planning Advice Note ( PAN) 63- Waste Management Planning. PAN 51- Planning and Environmental Protection, is also relevant in setting out the relationship between planning and environmental controls. Developers and planning authorities should have particular regard to the general principles and policies, and to the specific section in NPPG 10 on WWTW under the heading 'sewage treatment' (paragraphs 67-70).

The two primary methods of regulatory control of odours are Statutory Nuisance and IPPC. The controls applied by Statutory Nuisance are largely reactive (they only allow action where a nuisance exists, or is likely to exist or recur). However the powers under IPPC are proactive (that is they allow the permitting of processes by establishing conditions for all aspects of the design, operation and management of processes).

There is a long standing principle that the planning system should not be operated so as to duplicate the statutory responsibilities of other, more appropriate pollution control agencies. In these circumstances, where WWTW come under the control of IPPC it will seek to ensure that control measures are implemented to avoid the creation of odour nuisance. Where WWTW are not subject to IPPC control, the careful use of planning conditions to require inclusion of odour control measures and to establish operating conditions may be appropriate.

PAN 51 states that where the possibility that the release of smell might result in nuisance or loss of amenity from a proposed facility subject to planning control, this may be regarded as a material consideration for planning reasons. It would be good practice as part of the planning process for the WWTW operator to provide an Odour Management Plan ( OMP - see Annex 3) to ensure that odour emissions have been considered and also to enable the efficacy of control measures to be assessed.

There is also a need to carefully consider the proximity of proposed new development to existing WWTW. Encroachment of odour sensitive development around WWTW can lead to significant problems, as the occupiers of any new development will expect and demand high amenity standards, often meaning that WWTW become subject to complaint for the first time. A standard size of 'cordon sanitaire' would be inappropriate, given the individual nature of each WWTW; however, the operational and complaints history of a WWTW and other potential odour issues should be carefully considered before permitting new development in the immediate vicinity.

The role of odour modelling in assessment of new or upgraded WWTW and also in considering development in close proximity to existing WWTW is further discussed in Section 11.4.

Under the Environmental Assessment (Scotland) Regulations 1999, proposals for WWTW may require an Environmental Impact Assessment ( EIA) to be carried out in support of any planning application. Larger WWTW (in excess of 150,000pe) fall under Schedule 1 of the Regulations and therefore require an EIA. Smaller sites (in excess of 1,000 square metres area) are covered by Schedule 2 of the Regulations and would require to be screened to establish whether they were likely to have significant environmental effects. If this proves to be the case then an EIA is required. SEDD Circular 15/1999 suggests that where the site is in excess of 10 hectares or 100,000pe, although this would come under Schedule 2, a full EIA should generally be required. Where a planning authority decides that a statutory EIA is not required, it is still open to the authority to use its powers under article 13 of the General Development Procedure Order to request additional environmental information.

4.7 The Water Industry Commissioner and Price Controls

The post of Water Industry Commissioner for Scotland ( WICS) was created by Part II of the 1999 Water Industry Act and was established on 1 November 1999. The Water Industry Commissioner for Scotland is responsible for regulating all aspects of Scottish Water's economic and customer service performance and the primary role of the Commissioner is to promote the interests of customers of Scottish Water.

The Commissioner advises Scottish Ministers on the amount of revenue that Scottish Water needs to provide a sustainable service to customers and to fund its investment programme efficiently. The Commissioner does not decide the components of the investment programme (which may include investment for odour control), that is a decision made by Scottish Ministers.

The Water Industry (Scotland) Act 2002 created five Water Customer Consultation Panels ( WCCP) across Scotland to represent the views and interests of customers of Scottish Water in the areas covered by the Panels. WICS consult the WCCP during the price review process to take account of customers' views on investment plans and required service levels.

It should be noted that the roles of both the WCCP and WICS will be changing as a result of the Water Services etc. (Scotland) Act 2005.

5. WWT PROCESS OVERVIEW

5.1 General

The layout of a particular sewage treatment process will be wholly dependent upon the type of influent to the works, the location, the size and quality of receiving water. However, the following schematic in Figure 1 is a process flow sheet that covers the principal processes undertaken.

There are three principal functions of a waste water treatment plant:-

• Removal of pollutants, (mainly toxic material) and retention of re-usable material
• Treatment of water to permit safe re-use
• Treatment and disposal of the sludge.

The steps of a sewage treatment process are often divided into primary, secondary and tertiary. Primary treatment is largely a mechanical process to separate solids, secondary treatment is a largely biological process whilst tertiary treatment is polishing step for further purification possibly for specific contaminants. The main aim of treatment is to reduce biochemical oxygen demand ( BOD) and suspended solids ( SS) to acceptable levels. This is achieved by removing solids, and by aeration to satisfy the oxygen demand of the waste water, there being various methods of undertaking this operation. The removal of the solids and reduction of BOD produces sludge that can be recovered for beneficial land use after further treatment or sent for disposal.

5.2 Preliminary Treatment

Waste water entering the inlet works is usually screened to remove plastics, paper, cloth and other large debris. During periods of high flow the influent may be diverted to storm water tanks and this may occur before or after screening. Any influent diverted to storm water tanks will be processed as soon as flows return to normal. Effective management of storm water tanks is a key area in the reduction of odour. Screened solids are usually landfilled or incinerated.

Sand and similar heavy particles are removed next in a grit chamber. This chamber can be aerated to separate these particles from other suspended solids. The waste water spends a relatively short period in the grit chamber (in the order of minutes). The sedimented sand and grit is usually landfilled.

5.3 Primary Treatment

The finer solids are then removed in a settling or sedimentation tank, where the waste water spends a number of hours to allow the solids to settle or float and the sludge produced is scraped along the base of the tank for desludging. At this stage up to 70% of the solids and 30% of the BOD can be removed. The mechanical removal of solids as described above is usually called 'primary treatment', the sedimentation tank as the primary sedimentation tank, the overflow from the sedimentation tank as primary-treated waste water (primary effluent) and the sludge produced as primary sludge.

FIGURE 1 - WWTW PROCESS FLOW SHEET

5.4 Secondary Treatment

The activated sludge process is the most widely used biological process for waste water treatment at large and medium-sized works. The activated sludge in the aeration tank consists of flocs of bacteria, which consume the biodegradable organic substances in the waste water. This sludge is kept in the process by separation from the treated waste water and re-circulation.

The primary-treated waste water is passed to an aeration tank. Aeration provides oxygen to the activated sludge and at the same time thoroughly mixes the sludge and the waste water. Aeration is by either bubbling air through diffusers at the bottom of the aeration tank, or by mechanically agitating the surface of the water.

In the aeration tank, the bacteria in the activated sludge consume the organic substances in the waste water. The organic substances are utilised by the bacteria for energy, growth and reproduction. After the aeration stage the waste water enters a second sedimentation tank to separate the activated sludge from the treated waste water. The activated sludge is returned to the aeration tank. There is an increase in the amount of activated sludge because of growth and reproduction of the bacteria. The excess sludge is removed to maintain a desired amount of sludge in the system. This part of the treatment process is called 'secondary treatment', the sedimentation tank as secondary sedimentation tank, the overflow from the sedimentation tank as secondary-treated waste water (secondary effluent) and the excess activated sludge as secondary sludge. This sludge is usually returned to the incoming sewage flow entering the sedimentation tank and is then co-settled with primary solids to form co-settled sludge. At some works excess activated sludge may be kept separate for initial dewatering.

Depending on the flow rate of waste water, several parallel trains of primary and secondary stages can be employed. There are several ways to operate an activated sludge process. In a 'high rate' process a relatively high volume of waste water is treated per unit volume of activated sludge. The high amount of organic waste consumed by the activated sludge produces a high amount of excess sludge that may rapidly decompose and become highly odorous if not treated. In an 'extended aeration' mode of operation the opposite condition takes place. A relatively low amount of organic waste is treated per unit volume of sludge with little excess sludge to be removed. Removal of BOD is higher in the extended aeration mode compared to the high rate mode, but more waste water can be treated with the latter mode. The excess sludge from extended aeration is usually biologically stabilised and not as likely as high rate sludge to decompose and produce significant odour.

An activated sludge treatment process can be operated in batches rather than continuously. One tank is allowed to fill with waste water. It is then aerated to satisfy the oxygen demand of the waste water, following which the activated sludge is allowed to settle. The treated waste water is then decanted, and the tank is filled with a new batch of waste water. At least two tanks are needed for the batch mode of operation, constituting what is called a 'sequential batch reactor ( SBR)'. SBRs are suited to smaller flows, because the size of each tank is determined by the volume of waste water produced during the treatment period in the other tank.

The alternative to the activated sludge process is the use of a percolating trickling filter that is a method of secondary treatment widely used at small WWTW. This is a bed of solid media for bacteria to attach on its surfaces. Waste water is irrigated over a bed of graded mineral materialon the solid media (stones, waste coal, and gravel) or specially manufactured plastic media. As waste water trickles over the surfaces of the solid media organic substances are trapped in the layer of bacterial slime. The bacteria consume the organic substances in the same manner as in the activated sludge process, while air diffuses into the slime layer from the air spaces in the bed of the trickling filter. Growth and reproduction of the bacteria take place and result in an increase of thickness of the slime layer, particularly at the top of the biological filter. Periodically bacterial slime sloughs off the surfaces of the filter media and leaves with the treated waste water.

Solids derived from the sloughing off of bacterial slime are separated from the treated waste water in a sedimentation tank (often termed a humus tank). Sludge from this sedimentation
tank is not returned to the trickling filter but is usually returned to the inlet sewage flow for
co-settlement with the primary sludge for treatment and disposal as a co-settled sludge. The trickling filter and associated sedimentation tank is also termed 'secondary treatment'.

5.5 Sludge Processing

One of the key operations is the final treatment of the excess sludges produced in the process. The purpose of the sludge process is to reduce the liquid content of the sludge and volume to minimise downstream costs and stabilise the sludge to allow safe beneficial use for land conditioning or alternate disposal methods. The stabilisation process minimises the potential for odour generation and also destroys the pathogens.

The main processing stages are as follows (these may be used singly or in any combination):-

a) The first stage will involve thickening to reduce sludge volume by 50 - 70%. This is carried out either by gravitational thickening in tanks or by mechanical means using centrifuges or belt-thickeners. Mechanical thickening requires the addition of a polymer conditioner to the sludge.

b) The thickened sludge is then stabilised by either biological or chemical means.

c) The two main biological methods are:-

• Mesophilic anaerobic digestion at approximately 35 ^oC. Anaerobic digestion of sludge has four main stages. In the first phase protein, carbohydrates and fats are broken down by hydrolysis to form amino acids, sugar, glycerine and fatty acids. In the second phase acid fermentation occurs producing some fatty acids and alcohols. In the third stage acetogenic bacteria covert organic acids into a mixture of propionic and acetic acids plus hydrogen gas. In the final stage methanogenic bacteria convert the hydrogen and carbon dioxide to methane.

The key characteristics of anaerobic digestion are:

• Solids can be reduced by up to 50%
• The high ammonia content of digested sludge is suitable as a fertiliser due to the readily available nitrogen
• Pathogens are substantially reduced
• Sludge is less offensive in odour and appearance
• The gas produced (digestor gas) is a useful fuel and can be used to heat the digestor and also to generate electricity often through Combined Heat and Power ( CHP) systems (municipal waste water sludge can produce approximately 500m ³ of gas per tonne of dry volatile solids).
• Thermophilic anaerobic digestion (although this technique is not currently applied in Scotland)

d) Chemical stabilisation is relatively straightforward and is quite often used at smaller waste water treatment Works. The stabilisation is achieved by the addition of lime (or other alkaline materials) to raise the pH of the sludge to inhibit the growth of micro organisms (typically to pH 12 for 2 hours). In order to meet the pathogen removal required for re-use the material is usually held above 55 ^oC for a period of time.

e) Mechanical Dewatering to produce a solid cake of 25 - 50% solids. Dewatering may be achieved by filter pressing, centrifugation or belt-pressing and it always requires the preliminary conditioning of sludge with polymers. Sludge cake may be used in agriculture, disposed of to land or used to feed a thermal dryer.

f) Thermal drying to minimise volume is the ultimate stage of sludge processing and is carried out by various types of heated drying systems. The final water content of the product is less than 10% and the overall volume reduction compared to the original liquid raw sludge volume can be greater than 99%.

6. WHAT CONSTITUTES AN ODOUR NUISANCE?

6.1 What is an odour?

The subject of odour is a complex one. The response of an individual to exposure to an odour is subjective - how strong is it, what does it smell like and how often/when does it occur and in what context? The following characteristics further complicate the assessment of odour:-

• An odour can arise from a single substance or from a combination of substances.
• In combination with other substances, the characteristic odour of a single substance can be modified so as to be unrecognisable.
• Odour from a combination of substances changes as the mixture becomes diluted and the concentration of each component falls below its odour threshold.
• Odours from a substance or mixture of substances can be pleasant when dilute or offensive when concentrated.
• Odours that are pleasant or acceptable to one person can be offensive to another.

6.2 What is an odour nuisance?

The key to understanding the odour control principle of Statutory Nuisance is that the presence of an odour itself does not necessarily constitute a nuisance. The characteristics of an odour that are taken into account when assessing nuisance are odour type (pleasantness or offensiveness), odour strength, frequency and duration of release, persistence in the environment and the extent of interference with enjoyment of the amenity of a neighbourhood. There are also cases when although odours are present, the control measures put in place by the operator fulfil the test of best practicable means (bpm) or are present due to circumstances beyond the control of the operator (such as unforeseen breakdown, adverse weather conditions).

When assessing the levels of polluting substances necessary to avoid harm to health it is usual to determine appropriate numerical values for such limits. In the case of odour, the response of the human nose means that each individual will make his or her own subjective assessment as to whether the odour is offensive and whether it is considered to be acceptable.

Whilst it is possible to measure the odour strength using a standardised method (dynamic olfactometry as detailed in BSEN13725), it is more difficult to quantify the offensiveness of the odour. Where numerical rankings are used to try and simulate the sensory annoyance, they still rely upon subjective analysis and hence standardisation is almost impossible.

In general, odour effects are not caused by one single pollutant or chemical species, odour is a 'cocktail' of chemical species emitted from a process. The nose is an extremely sensitive receptor of odour - it can respond to small variations in concentration over periods of a few seconds and at concentrations of fractions of a part per billion. There are many issues that influence the perception of an odour including variations due to the subjectivity of the receptor, dispersion of odour due to local meteorological conditions and variations in the generation of odour from the process due to raw materials and cycle operations in the process.

In general, there is very little difference between the offensiveness of an odour and its potential to cause nuisance. The assessment of offensiveness of odour remains a subjective sensory olfactory response of observers. However, all odours have the potential to be offensive and cause annoyance.

The following matters should be considered when determining the degree of potential offence or the existence of an odour nuisance.

NATURE - this refers to both strength and character of the odour. Odours that would be generally accepted as 'unpleasant' will be potentially offensive. Odours from a sewage process would generally be accepted as more unpleasant in comparison to odour from, for example, a bakery. There are methods of qualitatively assessing 'pleasantness' of odour such as the use of the Hedonic scale, but these are still very subjective. Odour can also be described by a subjective descriptor such as 'sweaty', 'faecal', 'fishy', 'spicy', 'fruity' etc. The strength of an odour referenced to its detection threshold can be quantified and the higher the odour strength, the more the likelihood of an odour being detected. If an odour is present above the threshold of recognition, this will usually lead to the receptor being able to clearly identify the odour and often associate the odour with potential sources or activities. The ability to measure odour concentration often results in this being a major factor in the assessment of an odour problem.

FREQUENCY - odours that are released frequently or continuously from the process are more likely to be determined to be a nuisance. However, in some circumstances odours that are released periodically can be more intrusive and the odour frequency is often assessed in conjunction with the odour's persistence in the environment.

PERSISTENCE - odours which are continuously released from processes or those which are emitted on a frequent basis but persist in the environment for a long period (that is do not readily disperse to a level where the odour is no longer detected) are more likely to be judged as a nuisance. It is possible to put forward a case that even less unpleasant odours (such as food processing odours) may be offensive if the releases are continuous or frequent and persistent. The persistence of an odour is also affected by the meteorological conditions.

METEOROLOGICAL CONDITIONS - as the majority of odour control techniques finally rely on dispersion for minimisation of odour effects, the meteorological conditions will be of prime importance. If conditions exist that are disadvantageous for dispersion, odours may be detected even though the best available control methods are in use. These conditions should normally prevail for less than 1% of the year. Thus in most cases, the detection of an odour which is potentially offensive will result in a detailed process assessment to ensure that the process management and control is operating normally and then to identify possible weather related effects.

LOCALITY AND SENSITIVITY - the potential for amenity interference is largely related to the character of the neighbourhood and the time that the odour occurs. The number of persons affected and the degree of intrusion will depend upon the proximity of the source and receptor. Odours are often subjectively more annoying during periods when members of the public are outdoors (for example daytime periods during summer months).

The assessment as to whether an odour is a nuisance therefore involves the investigation of many characteristics of the odour, the odour source and its causation. The determination of nuisance by an independent body, normally the local authority regulator, involves the collection of information and a balanced view as the degree of interference or annoyance the odour causes.

Chapter 7 below summarises the likely odour sources at a WWTW and Chapter 8 details the mechanisms that can be used to collect data and assess nuisance.

7. CHARACTERISING ODOUR FROM WWTW

The primary odours from WWTW are the result of biological degradation of organic matter by microorganisms under anaerobic conditions. The development of anaerobic conditions in sewage is often referred to as 'septicity'. Septicity can be onset by elevated temperature, high BOD, high sulphate levels and the presence of reducing chemicals. Anaerobic activity leads to the production of methane and hydrogen sulphide (H ₂S), ammonia ( NH₃), organic sulphur, thiols (mercaptans), amines, indole and skatole. During the fermentation phase of anaerobicity, volatile fatty acids, alcohols, aldehydes and ketones can be produced.

However, odour which is not typical of anaerobic conditions can also be generated by other mechanisms in a treatment works including:-

• Volatile substances in the influent such as petroleum derivatives, solvents
• Air stripping of volatile compounds and odours particularly from industrial effluent often at inlet works or during aeration
• Aerobic odours - which are often described as a 'musty' odour
• Ammonia odour from reactions after liming of sludges or when sludges become re-wetted.

7.1 Hydrogen Sulphide

Hydrogen sulphide (H ₂S) is often highlighted as the cause of odour from WWTW. Hydrogen sulphide is a toxic gas and can be a health hazard especially in confined spaces. However, the concentrations typically encountered around WWTW are substantially below the levels at which health effects may occur. Whilst hydrogen sulphide may be a principalcomponent of the odour cocktail, there are other compounds that cannot be ignored. Because it is relatively easy to measure, H ₂S is often used as a target indicator for odour. However, it is recognised that H ₂S is not a good indicator for industrial effluents, secondary treatment odours and dryers/incinerators, as it is proportionally less important as an odorous component in these sources. Also, based upon observations, the odour threshold for a sewage treatment odour is frequently 5 times larger than the value that would have predicted based upon the H ₂S concentration alone. Therefore whilst it is a valuable indicative target pollutant, careful evaluation of data from H ₂S measurements alone is essential.

7.2 Odour Components

There are many chemical species which have been detected in WWTW odours. In addition to hydrogen sulphide and other pollutants such as ammonia, there are a wide variety of organic sulphides and organic nitrogen-based compounds along with some oxygenated organic compounds and organic acids.

In addition to these compounds there are many potential substances which may be released depending upon the quality of the influent, for example if it includes industrial effluent. The range of contaminants potentially present in industrial effluent is extensive but those that are likely to be of concern are already odorous liquids (such as waste water from food production), warm effluent which may accelerate anaerobic conditions and volatile organic compounds such as solvents and petroleum derivatives.

7.3 Odour Sources at a WWTW

Whilst this CoP is focusing on WWTW and does not review the issue of odour in the sewerage transport system, the conditions within the drainage system will have a significant impact on the odour generation of the process due to septicity. The odour sources at any particular plant will be specific to that site and operation, however, the following are key sources which should be reviewed at all WWTW:-

• Inlet works - strong odours in influent may be affected by unfavourable sewer conditions (long retention times, brackish water infiltration, poor maintenance, industrial discharges) and long pressure mains - also the inlet works effectively vent any sewer gases
• Imported sludge and septic tank waste - the off-loading and storage of such materials prior to treatment can lead to odours
• Storm water storage - usually due to storage for excessive periods leading to septicity or due to infrequent or insufficient flushing of the tanks after emptying
• Primary settlement - highly odorous feeds or excessive sludge accumulation which goes septic - emissions can be caused by excessive turbulence of waste water at overflow weirs
• Secondary treatment - if highly loaded or odorous feed
• Storage and treatment of sludge - especially non-stabilised sludge
• Biogas leaks (from anaerobic digestors and gasholders, and at the first point of discharge of digested sludge
• Odours can be transported through the system and become airborne at turbulent locations - recycling can increase odour (such as sludge thickening).

Based upon the approach taken above to evaluate the odour potential of liquids and sludges throughout a process, Table 1 below provides a ranking for odour potential at various stages of the waste water treatment process.

TABLE 1 - ODOUR POTENTIAL FOR A RANGE OF OPERATIONAL STEPS

SOURCE	ODOUR POTENTIAL
Raw influent - typical	vv
Septic sewage from a rising main	vvv
Industrial effluent	vvv
Septic tank and sludge import	vv
Stormwater storage	vvvv
Stormwater tank desludging	vv
Primary tank feed	vv
Primary tank overflow	vv
Final effluent	v
Raw sewage sludge	vvvv
Liquors from raw sludge processing	vvvv
Digested sewage sludge - fresh	vvv
Digested sewage sludge - after storage	vv
Digested sludge filtrate	vv
Gravity thickener overflow	vvv
Released digestor gas	vvv

KEY:-
v- Low
vv- Medium
vvv- High
vvvv- Very high

7.4 Assessing Odour Impact

Impacts can be determined by a number of methods, however many of these only assess the effective odour concentration and do not take account of the other parameters which are involved in determining potential nuisance of an odour. In addition, direct measurement of ambient odour concentrations is not practicable.

There are a number of tools that can be helpful when assessing impact, including:-

Qualitative - see also Section 8.1

• Public complaints monitoring - both level of complaint and nature of the odour described. Further details are included in Annex 1.
• Local authority subjective assessment
• Correlation of complaints records with process operations at the time of complaint - use timing of complaints and odour descriptions as well as process data
• Olfactory screening - further details are included in Annex 2

Quantitative - see also Section 11

• Direct measurement of emissions at source - most frequently emissions are sampled and analysed by dilution olfactometry to ascertain odour concentration. It is also possible to measure the concentration of indicative chemical species.
• Measuring the odour destruction efficiency of abatement equipment
• Monitoring flow from ventilation systems serving enclosed or contained sources
• Estimating potential emission by bulk material analysis to determine odour potential
• Estimating odour emission rates based upon concentration or odour potential and flow
• Odour Dispersion Modelling - see Section 11.4.

In the final analysis the determination of acceptability of odour management will be based upon sensory assessment in the area surrounding the process.

8. INVESTIGATION AND ASSESSMENT OF COMPLAINTS

8.1 The general approach to complaint assessment

This section deals with the complaint assessment procedures to be adopted by local authorities. The primary role of this assessment (as detailed in Steps 1 to 7) is to ascertain whether an odour that is the subject of complaint constitutes a Statutory Nuisance. There is additional guidance on the methods of complaint investigation appropriate for WWTW operators in section 12.4 that is focused more on identification of causation of the odour and control options. However, it is important to stress at the outset the need for the local authority and WWTW operator to share information throughout any investigation.

There are many tools that can be used in the investigation and assessment of odour complaints. Sometimes, the problem may be quite simple to deal with, some of the steps will be obvious, and the whole process through to resolution may be fairly intuitive. At other times, the problem may be more complex and a step-wise approach can help clarify for all stakeholders the route through to resolution of the nuisance.

The flow chart in Figure 2 summarises the steps and issues involved in the investigation of odour nuisance complaints by local authorities. It is important that all steps and decisions are documented in order to justify the measures chosen to resolve the odour nuisance. The first stages (Steps 1 to 7) are the key steps in determining the existence of a Statutory Nuisance and are expanded below. Steps 8 and 9 (identifying actions) are further detailed in Chapter 9 and Part 2 of this CoP.

Step 1 - Complaint received

The local authority receives a complaint alleging potential odour nuisance from a WWTW.

Step 2 - Share information

The primary reasons for investigation of complaints are to assess potential nuisance and identify the likely cause and source of the odour. It is essential to utilise the expertise of the process operator at this stage and therefore complaints should be forwarded to the operator without delay. This notification may be by telephone, fax or e-mail but the system should ensure that a named person at the WWTW is the contact and be able to carry out an assessment without delay. In cases where sites are not manned, the local authority should agree a local point of contact with the operator.

It is also good practice for the operator to be involved in liaison with the local community. This is likely to give operators a better understanding of the intensity, scale and emotive nature of any problems arising from their site. It is also likely to better motivate the operator to cure the problems and make them, perhaps, more accountable for the consequences of their development and the controls they exercise upon it. By inclusion of regular site visits within this liaison, it is possible also for the community to feel involved in the improvement of the amenity in the locality where problems are seen to be being overcome.

Step 3 - Determine what to record and how

The local authority will already have a general record keeping and complaint registration format and this should be adapted to avoid duplication. There are some basic details which should be recorded. These include:-

• Complainants details
• Odour source (if known)
• Time of complaint
• Complainants description of odour - including a subjective description of the odour and strength (possibly by comparison with other odours), location and extent of the odour and date, time and duration of the episode
• Weather information
• Any other information the complainant can offer on activities at the alleged odour source.

An example of an odour complaint form is included as Annex 1 of this CoP.

Step 4 - Who to involve

The local authority will obviously involve complainants and the operator of the works in their investigations. However, the local authority will need to determine how widely they seek to involve the various stakeholders such as councillors, residents groups, MSPs and SEPA in their investigations. The number, frequency and geographical spread of the complaints along with the intensity of complaint will largely determine the involvement of councillors and MSPs. It is essential that the operator and the residents are involved in the whole process to ensure that there is transparency and also to establish at the outset clear targets and goals for determining success of control measures.

Step 5 - How to respond

At the outset of any investigation, the local authority should assess realistically how quickly they will be able to respond to complaints and hence the approach which they will use for investigation. If they are able to attend rapidly after a complaint they may be able to carry out effective appraisal of the complaints independently by subjective assessment. If they are not able to respond to complaints at the time that the odour exists, it may be necessary to use the members of the public as indicators of the extent and nature of the odour. This is further reviewed in Step 6 below.

It is important to remember that whoever carries out the assessments, their individual sensitivity to the odour should be determined.

Step 6 - Identify the source and cause of complaint

In order to successfully resolve odour problems, it is essential to fully understand the source and cause of the odour and the operational conditions that lead to the complaint. A common strategy to monitor odours should be devised by consultation between the local authority and the WWTW operator to allow comparisons between the operator's records and those of the local authority.

The first step in the investigation is to select the most appropriate methodology for assessment. There are many techniques, but those commonly applied include:-

• The use of complaints records - timing of complaints, odour descriptions and process operations at the time
• Sniff testing - this is effectively a process site inspection trying to identify and characterize the odour quality and source
• Complaint location assessment - walking around the location of the complainants alleged odour impact allows the matching of the complainants descriptions to those assessed by an independent assessor

Operator process records (including maintenance and breakdown conditions) are an invaluable source of information about process conditions at the time of complaint - this would allow odour trends to be identified and possibly reconciled with particular process operations and maintenance or influent types.

Further guidance on undertaking odour assessment and a specimen record form are included as Annex 2 to this CoP.

The purpose of the assessment is to determine the source and cause of the odour. It is often possible to identify correlation between odour episodes and operational conditions. In order to evaluate the possible sources and causes, in addition to inspections and assessments, the local authority will need to utilise the following:-

• Expert knowledge of operator or possibly consultants;
• Knowledge of plant operation conditions (especially problems) at time of complaint;
• Investigations, such as observational investigations and engineering investigations;
• Whether the WWTW itself is the source of the problem, or whether the odour results from a problem further upstream:
• Monitoring may be appropriate in some circumstances, usually if the source cannot be identified by any of the preceding techniques or if a high level of certainty is required. Monitoring of source odour releases (sampling and analysis) is covered in Part 2 of this CoP.

Step 7 - Does the odour constitute a Statutory Nuisance?

The key to understanding the odour control principle of Statutory Nuisance is that the presence of an odour itself does not constitute a nuisance. The characteristics of an odour which are taken into account when assessing nuisance are odour type (pleasantness), odour strength, frequency and duration of release, persistence in the environment and the extent of interference with enjoyment of the amenity of a neighbourhood as outlined in Section 5.2 of this CoP.

Where a site is shown beyond all reasonable doubt to be the cause of odour problems in a locality, all relevant potential sources of odours on the site should be identified and examined in more detail. Not every potential source of odour on the site may be generating offensive odours at any one time, but all should be assessed in view of their potential detriment to amenity. There may be other sources of offensive odours in the locality which should be taken into account during any investigation and these too may need to be addressed by the local authority.

FIGURE 2 - ODOUR INVESTIGATION FLOW CHART

The following are issues that should be taken into account to assess the validity of complaints and attempt to identify sources and causes for each complaint:-

• Whether each complaint has been independently 'qualified' by a local authority officer and the "quality" of a complaint (hypersensitive individuals, vexatious complaints, etc);
• the volume of complaints against the alleged nuisance and the geographical spread of complaints - mapping of the complaints and odour assessments of the local authority may help with this assessment;
• Is odour nuisance a one-off event or a regular occurrence - trends in exposure can be verified to some degree by complaint and process records but may involve asking complainants to keep odour diaries or even undertake a survey to determine the attitude of the local population to the odour
• knowledge of potential sources on the WWTW (do complaints correspond to plant problems or activities, are they influenced by wind direction in relation to the WWTW, complainant and the distance of complainant from site).
• knowledge of potential sources other than the WWTW (again whether these correspond to complaints and wind direction in relation to the WWTW, complainant and the distance of complainant from site).

The locality of a process site will influence the assessment of the potential for odour nuisance. In cases where the site has a low odour impact due to its remoteness from sensitive receptors and the escape of odour beyond the site boundary would be unlikely to cause nuisance, the standards of control required will be lower. Assessment of the potential for odour impact beyond the site boundary should take account of all predicted wind directions and weather conditions which are typical of the location in question.

Step 8 - Consider whether the WWTW Operator is using bpm

In the determination of the next steps in cases where the local authority consider that an odour nuisance exists, it is recommended that the local authority assess whether the control measures put in place by the operator fulfil the test of best practicable means (bpm). The possibility that the nuisance is due to circumstances beyond the control of the operator (such as unforeseen breakdown, adverse weather conditions) should also be considered.

In circumstances where an odour nuisance exists but the local authority consider that bpm is being used, the WWTW operator should be required to demonstrate to the satisfaction of the local authority that bpm has been applied. This may involve testing of emissions and investigation of plant design and operating conditions. If the WWTW operator is to undertake such investigations, it is recommended that the service of an Abatement Notice be withheld until these investigations are completed.

If bpm is being applied, it may not be in the public interest or effective use of public funds to proceed further with Statutory Nuisance action. In such cases, if the local authority is of the opinion that service of an abatement notice would afford an inadequate remedy, section 81(5) of the Environmental Protection Act 1990 allows injunctive action to be taken. Also, section 82 (1) of the Environmental Protection Act 1990 allows any person aggrieved by the existence of a Statutory Nuisance to seek an order from the sheriff to abate the nuisance and prohibit the recurrence of the nuisance themselves.

Part 2 of this CoP outlines some of the control measures that can be considered bpm for WWTW.

Step 9 - Identify and implement an action plan

Any plan detailing a programme of works to alleviate an odour nuisance must be fully explained to all stakeholders. If the local authority is satisfied that a Statutory Nuisance exists, they must serve an Abatement Notice. The terms of the notice can require abatement directly but it is recommended that in most cases, further investigation and a step-wise programme of work to resolve the odour nuisance represents the most effective approach.

The first step may involve the collection of additional data to allow a review of the available control options and develop a design specification for any control measures. In some cases the means may be obvious and very little information will need to be collected. Other cases will be more complex, requiring more information to tackle the problem successfully. Thus the amount of effort and detail in obtaining this information will vary depending on the severity of the problem, and the required certainty for confirming the source(s) of the problem and setting the abatement specification.

Next, the information collected will be used to make a judgement of how the previously identified source contributes to the odour nuisance and how much it needs to be reduced to prevent the nuisance. Again this may appear obvious in some cases and one can proceed intuitively based on very little information, e.g. for covering an open tank it is not necessary to work out a quantitative value for how much the emissions should be reduced - the assumption is made that the control measure will be close to 100% effective. Other cases are more complex or a greater level of certainty is required, for example, specifying an abatement system with a minimum odour removal efficiency. Considerably more effort/detail will be used here to assess the impact of the odour release taking into account the environmental pathways to the receptors.

Consideration should also be given to possible changes in the nature of odour due to the implementation of controls. For example if certain odorous compound is reduced below the odour threshold, other odours may become apparent as they are no longer masked by the original odour.

Section 9 below outlines the methods available to deal with the odour problem.

9. ASSESSMENT OF CONTROL MEASURES AND STRUCTURE OF PART 2 OF THIS CODE

9.1 The main approach

There is no single, absolute, technical fix that can be applied to all the different causes of odours from WWTW. There are many different means of preventing, controlling or reducing odours. It is possible, however, to develop an agreed plan of action that starts with developing the options and odour impacts and ends with the resolution of the problem. This plan of action should allow all stakeholders to see that the choice of control measures proposed for a specific site has been arrived at in a way that is both technically justifiable and takes into account the balance of benefits and costs. All stakeholders should be able to have confidence that the option chosen is appropriate to resolve the problem and is justifiable on cost-benefit grounds thus avoiding "gold plating" solutions. The Statutory Nuisance regime provides for a defence that the best practicable means (bpm) have been used to control nuisance odours. Within the context of this CoP, Part 2 provides guidance on the interpretation of bpm.

9.2 Aim of Statutory Nuisance controls

The intent of the Statutory Nuisance provisions is to ensure that WWTW apply adequate controls to prevent emissions where possible and otherwise to ensure that they are minimised and do not cause odour nuisance.

The locality of a process site will influence the assessment of the potential for odour impact - for example the location of a very sensitive land use such as a school or hospital close to a WWTW would result in a different threshold for establishing the presence of a nuisance compared to a commercial land use. It may also be necessary to include additional controls to avoid nuisance odour, for example where local meteorological conditions may more frequently lead to poor dispersion conditions. Whilst the absence of odour complaints beyond the process boundary does not equate to the absence of odour pollution, in cases where the site has a low odour impact due to it's remoteness from sensitive receptors, the operator should be required to implement less extensive odour controls according to the perceived risk.

9.3 Hierarchy of odour controls

Having identified the source of the nuisance, the WWTW operator should consider the different options that could be used to prevent, contain or control the odour. As a general principle, preventing odour releases is preferred to their control. Where it is not practicable to prevent the odour releases they should be minimised to a level that will not cause odour nuisance. There is a wide range of control measures that can be used, including:

• the general management of the WWTW (including influent and sludge management)
• the design, installation and maintenance of plant, buildings and structures
• the operation of the WWTW and its processes
• engineering solutions, e.g. containment, enclosure with venting and end-of-pipe

treatment (dilute and disperse or abatement)

Control options are usually considered in the following order of preference before escalating to the next level:

1. Site management and housekeeping

2. Operational and process changes

3. Containment

4. Enclosure with end-of-pipe treatment of excess air

However, practical, safety and financial restraints mean this hierarchy cannot be applied rigidly to every application and a cost-benefit judgement will determine the most appropriate choice for any given situation.

The approach outlined in this CoP is summarised in Figure 3 below.

9.4 Developing control measures

Sometimes, the problem may be quite simple to deal with, some of the steps will be obvious, and the whole process through to resolution may be fairly intuitive. At other times, the problem may be more complex and the step-wise approach can help clarify for all stakeholders the route through to resolution of the nuisance. The operator should document the decisions and findings of each stage so as to be able to justify the measures chosen to resolve the odour nuisance.

The amount of time, effort, detail and cost required for each step will vary according to the complexity of the problem. The response should be proportionate: sufficient to select the right means to stop the nuisance, but without making the process unduly lengthy or complex.

Implementation time of the control measure is an important factor and must be considered along with the effectiveness of the solution and its cost. It may be a deciding factor between several options of equal merit in terms of cost-benefit. Where the most effective control measure identified has a long lead-in time, the implementation of quick, low-cost high-benefit solutions should be considered as additional temporary measures.

By following an accepted and transparent cost-benefit assessment principle, all stakeholders are able to have confidence that the best practicable means was chosen to resolve the problem, but with some protection against "gold plating" for the sake of it. It is important when evaluating control measures that both capital and operating costs and environmental impact (such as energy use, chemical use and secondary pollutant generation) are considered.

9.5 Determining success

Where a plan for implementation of certain measures has been developed, the WWTW operator must document clearly the risks and triggers involved in the action, including:

• the criteria for successful resolution;
• when the next step in odour control measures will be taken; and
• the basis for such a decision.

It is essential that the local authority, in consultation with the WWTW operator and the other stakeholders (particularly the general public), establish and document appropriate criteria to determine when control measures have been successful. This is a very difficult aspect of the control regime and there are likely to be conflicting interests.

Determination of success will be on a site-by-site basis and indicative measures include:-

• reduced frequency of odour generation and detection at odour sensitive locations
• change in the nature of the odour (less offensive) which may be due to reduced intensity or a change in odour character
• reduced extent of area subject to odour complaint (the number of properties affected and geographical distance) - this should only be used for acceptance of interim, temporary or partial solutions .
• in general, using the number of complaints alone will not be an effective measure of success.

In addition to establishing indicative measures of success, the following guidance should assist local authorities is establishing a yardstick against which the performance of the WWTW operator can be measured:-

• subject to Section 4.4, all WWTW should implement the provisions of Section 12 in Part 2 of this CoP irrespective of whether they are subject to odour nuisance
• if these measures do not prevent an odour nuisance, a plan of action should be agreed with WWTW operator (and this may be formalised by service of an Abatement Notice)
• the plan should aim to implement the measures in accordance with the following hierarchy :-

1. Site management and housekeeping

2. Operational and process changes

3. Containment

4. Enclosure with end-of-pipe treatment of excess air

• at each stage an assessment of the impact of the measures should be made - this may involve surveys of the neighbourhood using sniff-testing or questionnaires to determine the attitude of the local inhabitants to the odour and their views on improvements
• it may be possible to carry out odour emission measurements before and after the implementation of measures (although this is difficult from fugitive sources) - further guidance in included in Section 11 of Part 2 of this CoP
• enclosure and extract ventilation rates can be measured and compared to good operating designs as detailed in Section 14 of Part 2 of this CoP
• Section 14 of Part 2 of this CoP details an odour abatement efficiency that can be used to determine the efficacy of abatement equipment.

Where WWTW operators find that there are competing demands for investment in odour measures at its different sites, it shall use a documented prioritisation process to ensure those sites with the most significant odour impact are targeted first. The prioritisation methodology shall take into account the number and severity of complaints at the different WWTW, but the number will not necessarily be the prime determinant of significance.

The Scottish Executive has established the Scottish Odour Steering Group ( SOSG) to oversee the production of the Statutory Code and advise on prioritisation of odour controls at existing WWTW. The Group includes representatives from Scottish Water, Scottish Executive, local authorities, SEPA, WICS and WCCP.