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

Part 2 Code of Practice on Control of Odour Nuisance from Waste Water Treatment Works

10. Concept of Best Practicable Means

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. There are also cases when although odours are present, the controls 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).

Part 2 of this CoP details measures that can be regarded as best practicable means for the control of odours from WWTW based upon the following approach:-

• To prevent the generation of odour where possible
• To contain odour and use effective treatment techniques to minimise releases
• To promote good odour control practices such as storage, cleaning, maintenance etc.
• To ensure that sensitive receptors are not exposed to offence to their olfactory senses.

The aim 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 nuisance to the human olfactory senses.

There is no single, absolute, technical fix than 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. Similarly, not all WWTW will require the implementation of all measures to avoid odour nuisance. Therefore the control measures should be carefully evaluated and implemented on a phased basis to ensure that only those works which are necessary to abate odour nuisance are implemented.

Also a WWTW operator may apply an alternative bespoke control measure based upon new or developing technology that has been demonstrated as suitable and effective for odour control from WWTW.

There will also be a difference in the options to control odours from new and existing works. Obviously, in the case of new works, plant and equipment locations can be selected based upon prediction of odour emissions (see Section 11) to ensure separation from odour sensitive receptors. However, in the case of existing works, changing the location of plant and equipment, making process technology changes and installing new plant poses greater engineering challenges and has larger cost implications.

11. Assessment of Odour Emissions

11.1 General

There are a number of methods for the assessment of emission rates and odour potential from WWTW. This Section briefly reviews the methods for direct measurement of odours and also methods for assessing emission rates.

11.2 Monitoring for Odour

The primary problem with the measurement of odours is that most odours are mixtures of compounds and knowledge of the chemical compounds present in a mixture does not necessarily give an indication of the human response. A subjective view - what it smells like to those who are actually exposed (ie what people may actually complain about) - can be obtained by using olfactometry and/or characterising the odour

The collection of meaningful samples of ambient air (e.g. at an affected area in the community, or at the installation boundary) for assessment by olfactometry is subject to a number of difficulties (even though a European standardized method EN13725 exists for the technique). The main problem relates to low concentration - generally too low for olfactometry - and so it is not commonly undertaken. However, it is a valuable tool for measuring the performance of odour abatement equipment and sources. In this case the odour concentration is measured on the inlet and outlet of the abatement equipment simultaneously and gives a very good measure of odour abatement efficiency.

Collection of samples for instrumental analysis is sometimes possible but fluctuation in concentration is often rapid and only direct reading instruments can give an indication of the concentration profile. A result that is averaged over a long period is rarely useful as it is the peaks which tend to cause annoyance, even if very transient.

The use of a surrogate for monitoring (namely H ₂S) has already been reviewed in Section 7.

11.3 Odour Potential and Emission Rates

Whilst it is possible to measure odour concentrations in the air, it is difficult to measure the odour emission rate for diffuse sources such as tanks and lagoons where there is no controlled flow of the pollutant from the tank. It is useful to be able to assess the likely odour emissions from diffuse sources and also the potential for a liquid to generate odour when analysing possible odour sources. There are two main methods for determining the likelihood of odorous emissions:

• Odour Potential - odour concentration in air that has been brought to equilibrium with the liquid sample by blowing air through the sample in a standard apparatus - ou _E/m ³
• Odour Emission Rate - this defines the total emission of odour from source and is expressed as the odour concentration multiplied by the flow rate - ou _E/s.

The odour emission rate can be used to review the relative importance of sources in terms of odour transport and can also be used in dispersion modelling. In cases where the flow rate and odour concentration cannot be measured directly for example because there is no vent point it is necessary to generate data by other means.

There are three methods for predicting odour emissions:-

• Floating Hoods/Lindvall Box - (for liquids without flows/static liquid surfaces) In this method, air is blown into the end of a box on the surface of an odorous liquid at 0.5 - 1.0m/s and a sample of the air at the outlet of box is taken for odour concentration measurement. The product of the odour concentration and airflow rate gives the odour emission rate. The method is relatively low cost but the sampling method does interfere with normal flow conditions in tank and hence alters the true odour emission.
• Micrometeorological measurements - measure the mean odour concentrations in ambient air at increasing distances from the source. Combined with wind speed, temperature and solar heating it will allow modelling of emissions. However, the sensitivity of test methods for ambient air monitoring means that this is only really applicable for strong odour sources.
• Extrapolation of Wind tunnel studies - allows data derived from studies to be used to model emissions based upon odour potential. It allows the impact of various control options to be reviewed and allows true emission modelling based upon measured or predicted odour emission rates.

Also, some of the emission estimation and impact modelling techniques have a particular role in assisting with the design and impact of new WWTW.

The first step in evaluating the potential for odour impact for existing works is to carry out a detailed process review to identify possible odour generation and process related controls. After this initial process assessment, the next stage is to identify where, how and why odour emissions occur and to measure or estimate emission rates. Specific compound analysis and odour potential measurement in the liquid streams will identify formation of odours whilst air samples can be taken to identify odour sources.

The use of odour or H ₂S mapping may help. However, whilst H ₂S is in general an indicator used to target odours from WWTW and it is possible to measure H ₂S down to around 1ppb instrumentally, the limitation of this approach is that it assumes that H ₂S concentrations and odour are proportional. Based upon observations, the odour threshold is frequently 5 times larger than the value that would have predicted based upon the H ₂S concentration alone. Also, odours from certain stages of the process may be less dependent upon H ₂S for example aerobic odours, stripped organics and ammonia.

The data shows that in respect of mass emissions of odour, often relatively low odour concentrations result in large mass emissions because of the size of the source area, such as the primary sedimentation tanks. The data could be used as input to a dispersion model to assess potential odour impact at sensitive receptors for each source.

11.4 Odour Modelling

Dispersion modelling is addressed in the draft Environment Agency guidance on odours H4 ¹ however there are a number of issues that should be noted. Firstly, most models are based upon Gaussian theory and are used generally to determine maximum ground level concentrations typically over 1-hour average periods. As odour response relies on variations in odour concentration over much shorter periods of time, the averaging period should be much shorter.

Small-scale meteorological variations mean that the peak values can be up to 10 times the average values over 1-hour for point sources. Secondly, the model only produces concentration data and this may not be a direct indicator of potential offensiveness or nuisance.

However, modelling has an important role to play in the design of new plant, the assessment of complex odour problems at existing works, assessment of odour generation and control option efficiency for new plant and the assessment of potential impacts on proposed development close to WWTW.

The absence of information on the complex make-up of an odour makes any dispersion model difficult to interpret - for example different odorous substances will have differing odour thresholds and hence during dispersion the perceived nature of the odour may change. Also some strong odours may screen the presence of other intensely odorous substances.

Analysis of the impact of a mixture of odorous substances has a number of difficulties, including:-

• The odour intensity for the mixture may be greater than any of the individual substances but less than the sum of their intensities. As the number of components increase in the mixture, the intensity of the dominant component begins to give a good indication of the intensity of the mixture
• There are no apparent synergistic effects between odorous substances
• The greater the number of substances in a mixture, the greater the suppression of the individual constituents
• The greater the number of components, the more difficult it is to identify individual substances
• Hydrogen sulphide is the least frequently suppressed constituent whilst isovaleric acid and skatole are the most suppressed
• The unpleasantness of mixtures is usually greater than that of the sum of individual components suggesting that models using a single odorant for predicted odour impact will underestimate the potential impact.

The use of boundary odour limits as absolute control values is not recommended due to the difficulty in effective ambient measurement and also the uncertain relationship between odour concentration and nuisance. Their use of ambient odour guideline values may be appropriate as a benchmark for desktop modelling studies when predicting impact of new plant and when evaluating variation in potential odour impact of differing equipment location. In addition they may again be used as benchmark values when studying the odour profile around existing works or when trying to correlate complaint locations with predicted odour concentrations. It must be stressed that the modelling output relates to odour intensity and not odour nuisance and therefore care must be taken in the practical application of this data.

The selection of an appropriate benchmark value would be for the WWTW operator to justify based upon the characteristics of the odour and the locality. There are a number of sources of benchmark values including the current Environment Agency Guidance value of 1.5 ou _E/m ³ as a 98 ^th percentile of hourly averages for more unpleasant odours and a Dutch standard is 0.5 ou _E/m ³ as a 98 ^th percentile of hourly averages. It should also be noted that not only are averaging periods in the order of 1-hour, but they are expressed as percentiles. The 98% percentile is the hourly odour concentration that is achieved for 98% of the year and consequently this value will be exceeded, potentially by very high concentrations of odour over short periods that may themselves be a nuisance by definition.

Whilst there is no preferred method for modelling odour nuisance, the use of models may be appropriate to indicate possible odour impact. In these situations it is recommended that the following principles are adopted:-

• The model used and the inputs to the model should be agreed between the local authority and the WWTW operator
• It is important to consider intermittent sources (such as the operation of storm tanks) when carrying out such assessments.
• The relevant comparative odour guide values and averaging times to be used for assessment should be agreed between the local authority and the WWTW operator
• A consistent approach should be maintained and therefore the experience gained from the previous use of various modelling techniques for WWTW should form the basis of any decision on model and parameter selection.

12. Baseline Measures Applicable to all WWTW

12.1 General

There are certain baseline measures that should be put in place at all WWTW, as a matter of good practice, to minimise the risk of odour nuisance occurring. These baseline good practice measures should be implemented regardless of whether complaints are received or not.

The basic odour control techniques which would be expected to be put in place at all new and existing WWTW include:

• select locations of major sources away from sensitive receptors at the design stage
• good housekeeping and raw material handling practices
• control and minimisation of odours from residual materials and waste (including imported sludge or septic tank waste)
• preparation of an Odour Management Plan (see Section 12.2 and Annex 3)
• maintaining the effluent aerobic other than in processes which are specifically anaerobic
• avoiding anaerobic conditions and prevent septicity
• containment of strong odour sources and treatment in odour control equipment.
• design and operation of the process steps to minimise odour, including:
• minimise sludge retention time in primary settlement
• consider avoiding primary settlement by applying extended aeration
• for new and upgraded WWTW, cover (or allow for covering at a later stage where odour effects are difficult to quantify prior to commissioning)

Other baseline measures that should be put in place at WWTW are described in this Section.

12.2 Odour Management Plan

An Odour Management Plan ( OMP) should be prepared for all processes. This is a core document that is intended to detail operational and control measures appropriate to management and control of odour at the site. The format of the OMP should provide sufficient detail to allow operators and maintenance staff to clearly understand the operational procedures for both normal and abnormal conditions. The OMP should also include sufficient feedback data to allow site management (and local authority inspectors) to audit site operations. An example of some of the issues to be considered is included in Annex 3 and this can be summarised as follows:-

• a summary of the site, WWTW, odour sources and the location of receptors
• details of the site management responsibilities and procedures for reporting faults, identifying maintenance needs, replenishing consumables complaints procedure
• odour-critical plant operation and management procedures (e.g. correct use of plant, process, materials; checks on plant performance, maintenance and inspection)
• operative training
• maintenance and inspection of plant (both routine and emergency response)
• spillage management procedures
• record keeping - format, repsonsibility for completion and location of records
• emergency breakdown and incident response planning including responsibilities and mechanisms for liaison with the local authority.

The Odour Management Plan is a living document and should be regularly reviewed and upgraded.

12.3 Good housekeeping

Lack of good housekeeping can result in elevated levels of residual odour, and at times more serious sources of odour. The majority of good housekeeping is, in any case, good working practice and additional costs for odour control are minimal.

Location of odour sources

So far as is practicable, sources of odour shall be located at positions on the site that are likely to minimise the odour impact on nearby receptors. Account should be taken of distance, prevailing wind direction and obstructions. In practice, this will often mean locating the source of odour as far as practicable from the site boundary.

Tanks

The build up of scum or foam on tank surfaces can at times lead to odour and should generally be avoided. (However, a stable scum layer can reduce odour in some instances, e.g. sludge storage).

Draining tanks for cleaning has been implicated as a source of odour complaints. This should be scheduled to minimise impact. Where practicable, appropriate chemicals should be used to minimise this impact.

Storage of sludge

Storage of sludge product on site should be minimised.

Storage of screenings and grit

Screenings should preferably be washed and 'bagged' and grit should be washed to reduce odour potential. Skips containing screenings and grit should be covered, and removed from site as soon as is practicable.

Spillages

Spillages are usually due to plant failure but sources of possible spillage should be considered and avoided at the design stage. Often, spillages involve sludge: an interruption to continuous sludge processing could lead to spillage from a storage tank or cause sludge levels to build up in settlement tanks, one of the known risk factors for odour at WWTW.

12.4 Odour complaints administrative procedure

The WWTW operator shall have in place a procedure specifying how any complaints will be administered and progressed. This will show who is responsible for dealing with the different aspects of the complaint and should be integrated in the Odour Management Plan, for example:

• who in the company and/or at the site are complaints to be directed to as a point of central contact
• who in the company and/or at the site has management responsibility for ensuring complaints are dealt with
• who in the company and/or at the site has technical responsibility for dealing with the resolution of any justifiable complaints
• who in the company and/or at the site is responsible for liaison with regulator and local stakeholders on progress (from acknowledgement of complaint to, where justified, resolution).

It is recommended that complaints that are made to the WWTW operator are forwarded to the local authority. The procedure for notifying such complaints should be detailed in the Odour Management Plan and it is recommended that the WWTW operator submits regular summaries of complaints but only notifies the local authority immediately when incidents are significant and further complaints are likely.

12.5 Plant performance, maintenance, inspection and operator training

Defra research found that some odour problems at WWTW had been due, wholly or partially, to problems with plant maintenance and proper operation of odour abatement equipment. These problems were said to be due partly to difficulties in operation, lack of training and poor after-sales service. Plant performance, maintenance, inspection and operator training are therefore crucial in maintaining the effectiveness of odour control measures.

Plant performance

Operators should ensure the good performance of all plant, both the main treatment processes and odour control equipment. The maintaining of an Odour Management Plan should help to raise the priority given to operating and maintaining abatement systems.

Reagents and consumables

Adequate supplies of reagents and consumables should be kept on site. Records should be kept of the delivery and usage of all chemicals and reagents, and these records should be used to minimise the risk of running out. Schedules should be prepared for the planned replacement of longer-lasting reagents such as activated carbon, dry scrubbing chemicals or bio-filter media, together with any monitoring which has a bearing on the suitability of these plans.

Planned inspection and maintenance

An effective, planned inspection and preventative maintenance regime should be employed on all odour-critical plant and equipment identified in the Odour Management Plan as impacting on odour. Important points are:

• A written maintenance programme should be included in the Odour Management Plan
• A record of maintenance should be made available for inspection
• All external pipework used for scrubbing liquor, condensate, steam, cleaning water, irrigation water and process liquid transfer should be leak-proof and protected against frost
• A method for forewarning the community on intended maintenance works that may lead to odours beyond the site boundary

Emergency breakdown response

The operator should prepare an Odour Management Plan documenting the response for emergency breakdown of odour-critical plant. This should include the foreseeable situations which may compromise his ability to prevent and/or minimise odorous releases from the process and the actions to be taken to minimise the impact. It is intended to be used by operational staff on a day-to-day basis and should detail the person responsible for initiating the action.

The plan should also include clear timescales for response to odour incidents.

The Odour Management Plan should include a list of essential spares for the odour control equipment. Where practicable, spares should be held for items liable to fail on odour-critical plant. The equipment manufacturer should recommend which spares are subject to wear and foreseeable failure and are critical for the correct operation of the odour abatement equipment (such as pumps, adsorption media, nozzles etc.) and these should be held on site. It may be acceptable for certain spares to be available on guaranteed short delivery if the absence of a supply at the site would not lead to complete failure of the odour control equipment or to odour nuisance. The local authority should be notified without delay where the WWTW operator identifies odours that may cause nuisance beyond the site boundary.

Competence and training

Staff at all levels having duties related to the management, operation, maintenance or repair of odour-critical processes and plant should be trained and competent and have documented training records. In order to minimise risk of emissions, particular emphasis should be given to control procedures during start-up, shut down and abnormal conditions. This CoP encourages training to be addressed as part of an Environmental Management System ( EMS). The operator should maintain a statement of training requirements for each operational post and keep a record of the training received by each person whose actions may have an impact on the environment. Training should include:

• awareness of their responsibilities for avoiding odour nuisance
• minimising emissions on start up and shut down
• action to minimise emissions during abnormal conditions
• procedures for advising key persons and recording episodes when emissions occur which are likely to lead to odour complaints.

12.6 Maintaining Effectiveness of On-Site Measures

i) Procedural and management systems

• Odour Management Plan - this formalises odour-critical management procedures, operative training, and operational procedures (e.g. correct use of plant/process/materials; checks on plant performance, maintenance and inspection). Specific reference should be made to methods for the control and management of works which are unmanned for all or part of their operating period.

• Maintenance, inspection and plant operator training - these are crucial in maintaining the effectiveness of odour control measures.

ii) Technical measures

• Monitoring of source emissions of odour or a surrogate - for controlled odour emissions (e.g. from stacks, vents, ducts and odour abatement plant) monitoring of the source emissions (or a surrogate quantity) can be carried out. Monitoring may be periodic (e.g. annually to check odour abatement efficiency) or continuously to give an instantaneous indication of performance, often linked to an alarm.

12.7 Checks beyond the Site Boundary

i) Procedural and management systems

• Complaints monitoring - the monitoring of complaints is an important method of indicating the effectiveness or otherwise of measures implemented to reduce nuisance due to odour. Complaints can be made either to the operator of the WWTW or direct to other bodies such as the local authority Environmental Health department.

ii) Technical measures

• Monitoring of odour at the boundary-fence/perimeter line - monitoring can range from very simple (e.g. "sniff" tests), to complex (e.g. sampling and analysis of specific odorous compounds, e.g. H ₂S). The technique used shall be sufficient to do what is needed, i.e. demonstrate continuing effectiveness. This is analagous with being fit for purpose. The "sniff" test is probably the most common technique for assessing the (continuing) effectiveness of odour abatement measures. It is simple and cheap, although very subjective. One technique that should never be used at site boundaries is standard olfactometry. This is not of sufficient sensitivity for ambient air samples.

iii) Population surveys

• Such tools can be used to help monitor and maintain the effectiveness of abatement measures. Surveys conducted by market research would be too expensive for continuing application. Odour logs and diaries are more appropriate in this case. It is possible for members of the public to typify odours by comparison to common household or everyday odours.

The WWTW operator shall have in place procedures to ensure feedback of checks on the effectiveness of odour control so that appropriate actions can be taken in response to problems. Continuing effectiveness of odour control shall be a standing item on the agenda of appropriate management meetings.

The WWTW should ensure there is liaison with regulator and local stakeholders on the continuing effectiveness of the control measures and any problems that have been encountered or expected.

13. Enhanced Odour Control Measures Applicable to WWTW

There may be circumstances where, having carried out the baseline measures specified in Section 12, the process is still leading to odour nuisance at sensitive receptors.

Some WWTW operators have found it convenient to group these control measures into two bands: a first basic set of actions that can be triggered quickly and inexpensively immediately following the complaint, and a second level of more extensive measures that can be employed if the basic actions are not successful in dealing with the problem (i.e. stemming the complaints). This may include, for example, modifying the process or installing abatement. This CoP encourages tiered approaches where quick fixes can solve the problem, or temporarily alleviate it during further work or investigations.

13.1 Odour Control Aims

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

The hierarchy for implementing control options is usually considered in the following order of preference before escalating to the next level:

1. Site management and housekeeping (largely detailed in Section 12 of this CoP)

2. Operational and process changes

3. Containment

4. Enclosure with extract ventilation and end-of-pipe treatment of exhaust air.

It may be possible that the process can meet the aim of preventing odour nuisance without the use of containment, or enclosure with end-of-pipe treatment. It is often possible to reduce odours by careful process evaluation and changes, for example, by process operation and configuration. However, in cases where the baseline good practice housekeeping and operational controls of Section 12 cannot avoid odour nuisance at sensitive receptors, containment of odours or enclosure of sources and treatment of odorous emissions is likely to be the key to cost-effective control.

In order for the methods available to deal with the problem effectively, it is necessary for the operator to have a number of things in place:-

• sufficient day-to-day control to minimise or contain any problems via frequent and regular full inspections of the site carried out by the operator;
• a scheme to monitor the extent of the odours and to detect when a problem has arisen or is likely to occur;
• techniques and equipment which are acknowledged as being effective need to be in place or available to deal with incidents as they occur;
• a requirement to take effective action in the event of offensive odours being detected.

13.2 New Plant Design

The incorporation of good design practice can greatly reduce the potential for odour releases and can also ensure that plants are provided with sufficient odour control systems to avoid offensive odours in the locality. It can also greatly reduce the costs of retrofitting odour control systems. The environmental drivers for upgrading existing waste water treatment plants offer an ideal opportunity to optimise plant design for minimising and treating odours.

Odour problems can occur at almost any stage of a WWTW depending upon influent and plant location, operation and design. However, the areas most commonly responsible for potentially offensive odour releases are the inlet works, primary sedimentation, high-rate secondary treatment processes and all stages of unstabilised sludge handling and storage. There are opportunities for the selection of process stages which minimise odour release or facilitate more effective odour control.

One stage in particular is primary settlement. As the tanks are usually large, there is a significant surface area to emit odours at this stage. It may be more effective to use a low-rate biological treatment step such as extended aeration of crude sewage or a high-rate process within a building to avoid primary treatment. The choice obviously has cost implications but is more likely to be determined by size of the works and other process considerations. During design it is not uncommon to use dispersion models for selecting plant and process location and also for evaluation of the need to cover primary tanks.

There is extensive guidance on the design of waste water treatment plants in BSEN 12255. In particular, BSEN 12255 - 9 of 2002 deals specifically with odour control and ventilation.

13.3 Developing an Odour Improvement Plan

Once the baseline measures of Section 12 have been implemented, an assessment as to whether an odour nuisance exists should be undertaken. The flow sheet in Figure 3 outlines the key steps in evaluation of the various stages involved in identifying appropriate odour control solutions. In order to undertake the assessment of odour and the development of a control scheme, the operator should carry out a systematic review and analysis of the odour sources and control options. The outcome of this review should be an Odour Improvement Plan.

The plan should identify the odour sources, measurement and assessment methodologies and process data recording. Where an odour nuisance exists, the operator should evaluate the sources and causes of the odour and develop a list of the available options to control the odour release. The approach should be based upon the control hierarchy in Section 9.3 and in the first case these measures should be based upon process optimisation and possibly process change. The various options should be evaluated in relation to both capital and operating costs, environmental impact of the control option (energy use, secondary pollutants and pollutant transfer, raw materials etc.) and the odour reductions predicted. Each of the control options should then be compared to identify those options that are readily available, offer effective odour reduction and offer a well-balanced cost and environmental impact. This information should then be subject to discussion with the local authority to agree an implementation and monitoring programme.

Once the relevant controls have been implemented, a further analysis of the potential for odour nuisance should be carried out. If the odour nuisance has not been prevented, there should be further review of available enhanced measures or containment and abatement systems using the same methodology as before. The process is iterative until all the nuisance is abated, all available control options have been implemented or substantial works that represent bpm have been undertaken.

The Odour Improvement Plan is a living document and should be regularly reviewed and upgraded. The operator may choose to integrate the OMP and the OIP.

13.4 Transport of sewage to the works

This CoP concentrates on odour control options for WWTW and will not specifically address the potential odour issues associated with the sewerage transport system (drains, sewers and remote pumping stations). Some of the measures in this case are outside the scope of this CoP (such as trade effluent) but the WWTW operator should still review the impact and controls possible in such cases.

However, if the influent to the treatment works is already septic and undergoing anaerobic activity, it will have significant impact on the WWTW. Therefore all sewers should include good design, operation and maintenance to avoid septicity. The guidance in European Standard EN 752-4 on the design of sewer systems to minimise septicity will assist in minimising anaerobic conditions.

Measures that can be taken to reduce septicity andminimise the retention time of sewage in transport under anaerobic conditions, include:

• minimise the length of pumped sections
• ensure that the slope of gravity sections prevents sediment accumulation
• minimise intermediate storage
• prevent seawater intrusion
• avoid siphons
• avoid untreated putrescible and warm wastes from industrial sources
• regular cleaning to remove accumulations
• improve ventilation
• if septic conditions are developed, chemical dosing may reduce the amount of odour - this includes adding oxygen, hydrogen peroxide, nitrate or ferric salts
• air stripping at the inlet works and treatment of the stripped air.

13.5 Inlet Works

In general the inlet works are potentially a considerable source of odour from incoming sewage particularly if it is septic sewage at the inlet, odorous imported wastes (such as septic tank emptyings), storm conditions and storage and handling of screenings and grit. In the case of WWTW that are subject to odour complaint, it is common to cover the inlet works and vent to odour abatement equipment. Measures that should be taken to minimise odour releases from this source, include:

• Regular cleaning and flushing of screens and influent channels
• Grit and screenings transfer and storage in a manner to prevent spillage. Ideally screenings after washing should be dewatered and bagged (or contained in a covered skip).
• Lowering discharge points to minimise turbulence and volatilsation of odours
• Balancing the flow of sludge liquors to even the load over the day
• Imported sludges to go straight to sludge storage tanks and not through inlet works

13.6 Primary Sedimentation

The principal odour sources in primary tanks are excessive turbulence in the inlet distribution channel or stilling chamber, the overflow weir and the tank surface. Minimisation of the sludge retention time in the primary tanks can reduce the odour. However, if there is anaerobic activity before or during the primary sedimentation operation, the size of these tanks can make them a significant source.

Measures that should be taken to minimise odour releases from this source, include:

• Pre-treatment of incoming septic sewage or possible chemical dosing with nitrate or iron salts
• Reducing hydraulic retention times,
• Improving desludging both in efficiency and frequency and regular cleaning of the tanks, sumps, scum and grease removal equipment - aim to ensure that sludge is not held on the base of the tanks for more than 1-hour
• Reduce turbulence at the weir overflow by reducing the drop height from the weir
• Recirculation of nitrified final effluent during low flow and avoiding the recirculation of secondary sludge

13.7 Secondary Aerobic Treatment

Ensure that conditions remain aerobic. Maintenance and inspection of the air diffusion system and liquid irrigation are of great importance. Measures that should be taken to minimise odour releases from this source, include:

For trickling filters

• Media should be kept wet and hydraulic overloading or blockage should be avoided
• Clogging or ponding of the filter as a result of organic overloading, inadequate aeration or mixing, blocking of aeration vents or media breakdown may result in anaerobic conditions and odours
• Avoid sludge and solids settling due to low turbulence in the liquor especially close to any recirculated sludge return

For activated sludge plants

• Increased aeration by methods which minimise the generation of aerosols (for example sub-surface diffuse aeration) and maintain the activated sludge flocs in suspension
• Shrouding of the mechanical aerators to reduce aerosol formation
• Covering the inlet distribution chamber and anoxic zone may be sufficient in cases where odours occur

13.8 Final Settlement and Tertiary Treatment

At this stage the effluent and sludges should be oxidised and provided sludge retention times are carefully managed, odour release should not be a problem. Denitrification may be a problem with fully nitrified effluents giving rise to rising sludge and surface solids. This can be avoided by minimising sludge retention periods in the final tank.

13.9 Sludge Handling , Storage and Thickening

Sludge and bio-solids handling are usually the most significant source of odour release and good sludge management is a key issue. All raw sludge and bio-solids will release odour largely dependent upon age. In general, sludge handling, storage and processing should be enclosed or covered and provided with ventilation to odour-abatement equipment.

Measures that should be taken to minimise odour releases from this source, include:

• Unstabilised liquid sludges imported to WWTW should be transported in tankers or (if in solid form) enclosed lorries and should be transferred to storage tanks which are vented to odour abatement equipment
• Sludge which has been lime treated can generate odour, particularly ammonia, and should be stored under cover to prevent odour generation (including avoiding re-wetting of sludge cake)
• Sludges should be processed (thickened, digested or dewatered) as soon as possible after generation as retention will lead to anaerobic conditions. It is good practice to minimise the potential storage of sludge before treatment and storage for unstabilised sludge should be limited to a maximum capacity of 24-hours production
• All tanks and plant for unstabilised sludge storage and processing should be enclosed or covered and vented to odour abatement equipment
• Replacement of lagoons and drying beds with mechanical dewatering plant will help minimise retention and contain odours
• Avoid open storage of sludges or sludge cakes

13.10 Anaerobic Digestion

The gas produced in an anaerobic digestor will be odorous. It should not be released to air in an uncontrolled manner and will only usually be vented untreated in the case of an emergency activation of a safety device. Normally the gas will be used as a fuel in boilers to heat the digestor or used for fuel a combined heat and power system. In some cases an excess of gas production necessitates the operation of the pressure-relief valve and burning-off the surplus through a flare. Measures that should be taken to minimise odour releases from this source, include:

• Routinely drain condensate traps to remove water and avoid back pressure
• Ensure that the digester system is balanced in respect of pressure to reduce emergency pressure relief operation
• If the gas is vented to a combustion unit for energy recovery, a stand-by flare should be provided in case of combustion system malfunction
• Regularly inspect the operation of the flare to check in particular that the pilot will light the flare even if the flare has been overloaded
• Avoid turbulence of the sludge after digestion
• Secondary digestors are often not covered and they can lose up to 10% of methane generated and obviously also any odour associated with the sludge. The operation of the primary digester should reduce the risk of odour generation at the secondary stage. In cases where the operation of the primary digester leads to odour release in the secondary stage, the secondary digester may require covering and venting to an odour and methane treatment facility (it is essential to consider possible explosion hazards associated with this)
• Covering of digested-sludge feed channels, mixing wells and overflow take-offs
• Regular inspection of the seals of floating gasholders
• Any covers or abatement equipment provided for this source will require careful evaluation in relation to safety and explosion control

13.11 Thermal Drying of Sludge

Thermal dryers release a large volume of water during the drying and there are two options for odour emission treatment:

• Maintain the conditions in the exhaust treatment plant to ensure that the moisture did not condense. This would result in a wet emission with odour control provided by thermal oxidation
• Condense the water and use a more traditional odour treatment system such as a biofilter, scrubber or adsorber. There would be concern that by condensing the moisture any solids present would be re-wetted and may liberate more odour and also the generation of potentially odorous liquors.

13.12 Storm Water

The problem with storm water tanks is if they are not emptied soon after filling they can go anaerobic. The other major issue relates to the frequency and efficiency of flushing and cleaning of the tanks after use. These tanks are very large and are rarely covered. Implementation of the following measures should largely avoid odours from the storm tanks:

• Storm tanks should not be used to increase the hydraulic capacity of the WWTW - they should only be used for storm conditions
• If the tanks are to be used for balancing influent flows under normal flow conditions, septic conditions should be avoided either by the use of chemical additives or by maintaining solids in suspension by controlled aeration
• The tanks should be emptied within the shortest possible time once the hydraulic load has reduced to allow the treatment of the storm flow and should be within no more than 72 hours. This requires that the design hydraulic load allows the storm water flow to be treated within this timescale
• The tanks should be operated on a system to ensure that tanks are continually refreshed to avoid liquor standing and also that the tanks are emptied in order of the age of the storm water within them
• The tanks should be desludged and cleaned as soon as possible after use. Allowing the sludge to stand in the tanks will rapidly lead to odour generation. Consideration should be given to the provision of automatic flushing and desludging equipment in these tanks.

14. Odour Containment and Abatement

14.1 Odour Containment, Plant and Tank Covers

The most effective way of controlling odour released during the various process stages is to either fully enclose the plant within a building or to provide localised tank covers. There has been some experience in England of total plant enclosure using what is often termed 'triple containment'. The design and operation of these plants requires the use of different compact process technologies such as lamella settling (almost 10% of the area for traditional settlement), re-aeration and dissolved air flotation rather than activated sludge processing primarily to reduce size. The selection of process stages will have a significant impact on both water quality and odour generation. It is therefore recommended that the operator justifies the selection of technology and controls at the planning stage. At the design stage of new or upgraded works, it is essential that systems are designed to be free from leaks and offer good source containment of odours.

Whilst these full enclosure techniques are available, they carry a significant cost and may not be cost-effective. A more traditional approach to containment is the use of ventilated buildings for certain plant and equipment and covers for tanks. In general the following sources will require containment at source and venting:

• sludge digestion plants, dewatering facilities and tanks
• entire inlet works (pre-primary stage) - low concentration large volume
• grit removal, coarse screens, skips (leakproof and enclosed).

In general it is not necessary to contain emissions from:

• primary tanks (may require covers in sensitive locations odour control but can often be sufficient by good management and maintenance)
• aerobic tanks (need to avoid excessiveaerosols from aeration lanes and aerobic tanks - these can act as an odour stripper and could be a health and safety problem)
• final settlement.

The design of covers is relatively straightforward, the main problem being one of engineering such large structures to be able to take load and making provision for inspection and maintenance. They have to be designed to allow for adequate support, to support wind, snow and personnel loads and to give sufficient clearance from process equipment and may have to incorporate walkways. The materials of construction need to be resistant to light and corrosion and are often constructed from either glass re-enforced plastic or aluminium. In addition to loads, the covers need to be designed to allow for bridge scrapers (can use rotating roofs), access, inspection and vents.

The following are some key design requirements:-

• Minimise head space under covers to reduce the volume of air vented due to displacement
• Any inspection hatches or access points should be sealed and any pipework transitions should be sealed
• The design of tanks and covers should minimise the need for regular access for maintenance and inspection as confined space entry systems will be required
• The vent volumes need to be adequate to ensure no odour escape and also to account for air quality inside the cover (occupational exposure, corrosion and explosion hazard).
• Ventilation rates will depend upon the exact process operations but for tanks the design flows are typically 0.5 - 12 air changes per hour based upon the empty tank volume or 120% of the maximum filling rate. In the case of thickener tanks, the volume may increase to 200% of the maximum fill rate
• The design will take account of the fill and empty rate, maximum rate of change in headspace, likely gaps and leakage, evolution rate of flammables to maintain <25% LEL for methane (10% is good design)
• Allowance should be made for emergency ventilation of the tanks
• One problem with tank covers is that they cannot be easily inspected therefore tend to be poorly maintained.

Additionally, guidance on the design of waste water treatment plants in BSEN 12255 advises designers to :-

• Locate sources requiring abatement close together to optimise abatement options and minimise costs
• Consider explosion risk, corrosion, access and health and safety.

14.2 Odour Abatement Equipment

The air which is exhausted from enclosures usually requires abatement to avoid odour nuisance. It is possible to establish performance criteria to reflect what constitutes best practicable means (bpm) in relation to abatement equipment. This can be specified as follows:-

Any odour abatement equipment installed on contained emissions (ventilation air from the process building) should have an odour removal efficiency of not less than 95%². Determination of the destruction efficiency should be by dynamic olfactometry based upon manual extractive sampling undertaken simultaneously at the inlet and outlet of the odour control equipment. At least three samples should be taken from both the inlet and outlet.

There is a wide range of odour abatement equipment that can be used to treat emissions of contained air from WWTW. There are many factors which will affect the choice of equipment including required odour removal efficiency, flow rate and inlet odour concentration, type of chemical species in the odour, variability in flow and load, space requirements and infrastructure (power, drainage etc.). The range of technologies available is detailed in the Environment Agency H4 Guidance Note on odour.

It is important when evaluating the most appropriate control technology to consider both total cost (capital and operating) and environmental impact (such as energy use, chemical use and secondary pollutant generation). Often operating costs are closely linked with environmental impact (that is costs for energy, raw materials etc.) and wherever possible the most environmentally sustainable technique should be selected.

As odour abatement plant capacity is usually tightly specified (little spare capacity), the assumption is that all other measures are being correctly used - covers, doors, chemicals replenished etc. This therefore becomes a key management issue that should be included in the Odour Management Plan.

The site layout may permit a centralised plant or due to locational constraints it may be necessary to use more than one system for example on the inlet works and the sludge process. It may be economical to provide a number of smaller biofilters for individual sources but if the selected technology is wet scrubbing it may be more cost effective to provide a single system. In some cases it may be appropriate to divide the odour streams and use different technology based upon the load and characteristicsof each system.

Table 2 below summarises the main types of abatement equipment and the odour abatement efficacy that may be achieved.

TABLE 2- ODOUR ABATEMENT

SYSTEM	CAPITAL	CONSUMABLES	EFFECTIVENESS
Biofilters	Moderate	Need space, fan energy, media replacement 3 - 5 years	High >95% - not able to rapidly adjust to changes in flow or load
Bioscrubbers	Moderate	Fan energy, effluent needs oxygenation	High >95% - can handle higher H 2S loads than biofilters
Activated sludge plant	Low additional	Needs fully aerobic sludge	90 - 95% for H 2S and NH3 ; may be ideal as a polishing stage
Wet scrubbers	High	Fan energy, pump energy, dosing chemicals and effluent disposal - high energy user	Single stage <80% but multiple stage - >98%
Dry scrubbing (carbon or impregnated media)	High	Media replacement is a high cost with strong odours, suffer with moisture loading	> 95% ; Widely used for passive sources. Need several seconds residence for treatment
Catalytic iron oxidation	Moderate	Low operating cost	Specific for H 2S - good for low flow high load
Thermal oxidation	High	Fan energy and support fuel	>98% ; good for dryer vents and VOC loads
Ozone	Moderate	Replacement of source and energy for fan and ozone generator	>90% on low concentrations - good for building vents
Counteractants and masking	Low	Replenishment of chemicals	Not an abatement method - may be suitable for short-term use

Experience in operation of peat and heather type biofilters has shown that they do not perform well when the flow or odour load from the process is variable although other media (shell-type material) appears to perform better for these applications. There has been a considerable amount of biofilter and bioscrubber equipment installed at WWTW. The units range in size from 75 - 435,000m ³/hr but are typically 1600 - 3000m ³/hr. The suppliers tend to offer 95-98% odour removal, 95-99.9% H ₂S removal and 300 ou _E/m ³ in exhaust gases.

The industry approach is that emission sources which exhibit strong odour peaks are best treated in wet scrubbers or carbon systems as some bio systems have been overloaded previously. It is increasingly common to have scrubbers on the sludge processing operations (often 3 or 4-stage scrubbers are used).