Scottish Government

9. ODOUR CONTAINMENT AND ABATEMENT

9.1 Odour Containment, Plant and Tank Covers

Code of Practice Requirement - Paragraph 12

(1) In cases where odour containment within plant and equipment is not possible and control is based upon fully enclosing the odour source within a building or providing localised covers or enclosures, the system shall be kept free from leaks and the extract ventilation rate shall be consistent with the efficient capture of emissions and maintaining a safe working environment.

(2) Where upgrading works are required to comply with the requirements of sub-paragraph (1), and an OIP is required by paragraph 10, a plan identifying the proposed measures and timescales for carrying out the works shall be submitted to the relevant local authority as part of the OIP.

9.1.1 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.

9.1.2 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).

9.1.3 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.

9.1.4 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.

9.1.5 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 and leakproof
• 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)
• Measures should be incorporated to prevent the risk of tank collapse due to under-pressurisation when the tank is being emptied
• 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.

9.1.6 Additionally, guidance on the design of waste water treatment plants in BS EN 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.

9.2 Odour Abatement Equipment

Code of Practice Requirement - Paragraphs 13 (1), (2) and (3)

(1) All odour abatement equipment that treats contained emissions (odorous air collected by extract ventilation from enclosed or covered tanks or channels, plant, equipment or a process building) shall have an odour removal efficiency of not less than 95%.

(2) Where the inlet odour concentrations are very low and 95% odour removal efficiency is difficult to demonstrate due to measurement reproducibility and equipment efficiency at low concentrations, the final discharge to air should contain less than 500 odour units/m ^3.

(3) Determination of the odour removal efficiency shall be by dynamic dilution olfactometry in accordance with EN13725 and shall be based upon manual extractive sampling undertaken simultaneously at the inlet and outlet of the odour abatement equipment and at least three samples shall be taken from both the inlet and outlet.

9.2.1 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 should have an odour removal efficiency of not less than 95%.

Code of Practice Requirement - Paragraph 13 (4), (5) and (6)

(4) Where odour abatement equipment that has been installed before 22 ^nd April 2006 fails to meet the requirements of sub-paragraphs (1) to (3) but emissions from that equipment are causing an odour nuisance, the equipment shall be upgraded to the odour removal efficiency specified in sub-paragraph (1).

(5) Where odour abatement equipment that has been installed before 22 ^nd April 2006 fails to meet the requirements of sub-paragraphs (1) to (3) but emissions from that equipment are not causing an odour nuisance, the use of that equipment shall continue to be accepted until the end of its reasonable operational life.

(6) Where sub-paragraph (5) applies:-

(c) available equipment must be optimised for odour containment and removal; and

(d) to monitor on-going performance, an odour removal efficiency shall be established based upon operating data.

9.2.2 However, the compliance requirements of paragraph 13 of the CoP allows odour abatement equipment existing on 22 ^nd April 2006 that does not meet the performance requirement to continue to operate until the end of its reasonable operational life provided that emissions from the equipment do not result in nuisance beyond the site boundary. The available equipment must be optimised for odour containment and removal and in order to monitor on-going performance, an odour abatement efficiency should be established based upon previous operating data. However, in cases where emissions from existing odour abatement equipment are leading to nuisance odours, the equipment should be required to be upgraded to the specified efficiency in paragraph 13.

9.2.3 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.). Odour masking agents or counteractants should not be used routinely for odour control - however, they may be appropriate for short duration predicted or planned discharges or temporary control.

9.2.4 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.

9.2.5 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.

9.2.6 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 characteristics of each system.

9.2.7 Table 3 below summarises the main types of abatement equipment and the odour abatement efficacy that may be achieved.

TABLE 3- ODOUR ABATEMENT

9.2.8 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).

9.2.9 The performance of odour control equipment should be monitored to ensure that the plant is operating optimally to control odour releases. This assessment should include inspections of the process, buildings and equipment to check that emissions are being contained and controlled.

9.2.10 In this respect the following forms of indicative monitoring may be appropriate:-

TABLE 4- ODOUR ABATEMENT MONITORING

9.2.11 The following is additional guidance on the information included in Table 4 above on monitoring of plant performance:-

1. In the case of thermal oxidisers, flares or combustion equipment the operating temperature of the system will need to be maintained above 1123K (850 ^oC). In the case of boilers, care is needed in their use for odour abatement as the operating temperature and residence time may not have been designed for odour abatement and there is the potential for quenching in the boiler. In addition, it may be necessary to establish a minimum firing rate for the boiler to ensure that the boiler conditions are always optimised for odour removal. The measurement of odour abatement efficiency of the boiler can be used to demonstrate the correct operating parameters of the boiler.
2. In the case of scrubbing equipment, it is likely that multi-stage scrubbing will be necessary to meet the odour destruction efficiency requirements. In order to optimise the performance of the scrubber, it is important to ensure that it is well designed (adequate gas/liquid contact), well maintained, that the odours are sufficiently reactive with the scrubbing liquor to remove the odour and also that the reaction products do not themselves produce a volatile odour. In addition, additives to the liquor need to be automatically dosed with control by pH/Redox (over-dosing can lead to secondary odours from the scrubber associated with the chemical reagent). The scrubber will require regular inspection to identify possible blockage by salts that may form when treating emissions from sewage treatment processes.
3. If a bioscrubber is used, it is important to ensure that it is well designed (adequate gas/liquid contact), well maintained and that potential odours from scrubbing liquor are well managed. The scrubber will require regular inspection to identify possible blockage by biomass. In addition it is probable that the pH of the liquor will need to be controlled as the microbial activity of the biomass will be adversely affected by high alkalinity (which is a potential problem with emissions from certain sewage treatment steps).
4. Biofiltration can be undertaken using packaged, enclosed biofilters or open biomass (such as peat/heather). If a peat and heather biofilter is used, it is essential to control the pH of the biomass as the microbial activity will be adversely affected by high alkalinity (which is a potential problem with the high levels of ammonia). In this case it may be necessary to pre-treat the emissions for example by water scrubbing (this will also have the beneficial effect of humidifying the air and possibly reducing particulate loading). In order to optimise the performance of the biofilter, the biomass must be maintained below 30 ^oC (which may require gas cooling), must be kept moist, must have a gas flow at all times and leakage through edges and fissures must be avoided. Where water spray towers are used for cooling of waste gases before biofiltration, care must be taken in managing the liquor to avoid excessive microbiological activity and secondary odour generation. Biofilters will require regular treatment to overcome consolidation - this may be regular surface turning or deconsolidation by digging-out the bed.
5. Catalytic iron filters, adsorbers of more than 250 litres adsorbent capacity and dry scrubbing equipment should be regularly inspected and monitored continuously to determine hydrogen sulphide and odour breakthrough.
6. The required residence time for the biofilters will depend upon many design conditions and the media selected and will have to be sufficient to meet odour destruction efficiency. However the recommended residence time for peat and heather filters is a minimum of 60 seconds. It may be appropriate to provide a number of smaller biofilters rather than one large bed to achieve more even gas flows throughout the filter. This will also provide standby facilities in case of breakdown or failure of one bed if the biofilter capacity is designed for this purpose.

9.2.12 In addition to the continuous indicative monitoring outlined in Table 4, the odour control equipment should be inspected at least once a day to verify correct operation and to identify any malfunctions. This inspection should include:

1. identification of any leaks in air handling equipment and ductwork
2. in the case of scrubbing equipment, thermal oxidisers and other combustion equipment, the verification of the operation of the continuous monitoring equipment, identification of any blockages and also of any leaks of either odorous air or liquid.
3. in the case of biofilters, identification of any cracking of the surface or voids in the bed, leaks around the edge of the filter or air handling equipment, review of the moisture content (considering both flooding and drying out) and looking for signs of compaction or uneven flow.