Preventing odour nuisance
Pat Coleman of Black & Veatch investigates key odour control issues that need to be taken into account when designing new or upgrading current WwTWs
The neighbours might not know much about wastewater treatment - but they can smell when there is a problem. Nuisance abatement efforts therefore need as much attention as water quality issues when building new or upgrading existing plants. Many WwTWs were built in the 60s or 70s, often beyond the urbanised area. Things have changed dramatically, however, and many sites have seen encroachment of both domestic and commercial development. From the outset, the design of new plants or significant extensions presents challenges not only in the areas of effluent and sludge quality, but for a broad spectrum of environmental issues, of which odour management is one of the most sensitive elements. These challenges can only be met if odour management is integrated with the architectural and process design from the start. One of a new work's aims has to be to prevent an odour nuisance to current and future neighbours by strict emission limitation targets. For instance, such a design would achieve five odour units (OU/m3) as a 98 percentile at the nearest receptor, often at the works' boundary.Design process
The first step in the design is to identify what contributes to odours deemed offensive by the public. The conventional approach is to control emissions of volatile organic compounds (VOCs) and other gases, typically hydrogen sulphide (H2S) generated from the reduction of organic wastes in the absence of oxygen. These processes can both occur in the network and in the plant.
However, this does not account for all the types of odour considered offensive. Volatile compounds generated by aerobic activity have a sweet earthy odour which, in certain concentrations, can be as offensive to people H2S or mercaptans. Although the emission per unit area from an activated sludge plant is low, the odour quantity discharged is significant because of the large process surface area. Emissions from aerobic processes therefore need to be included in the dispersion models to ensure a comprehensive odour mitigation. Odour modelling and standards. A commonly used model for atmospheric dispersion is Complex-1, which produces a probability of exceeding a certain hourly average concentration at exposed locations. When an annoyance criterion is applied to these results, an odour unit contour can be plotted, which identifies the area in which odour nuisance may occur.
The model's results are used to set a discharge standard for odour abatement and locate the stacks within the site so they have the least impact on the site's neighbours. The designshould take into account a precedent set by case law (Newbiggin-by-Sea, 1993), which accepted in the absence of air quality standards for WwTWs in the UK, it is reasonable to plan using the 5OU/m3 - 98 percentile of 1h average concentration contour.
When the odour concentration exceeds five units at this contour, then annoyance can occur for a normal population. On the other hand, an uncertainty exists in the law relating to UK odour nuisance. Two processes of consultation are currently under way, which may result in the imposition of an even tighter standard. The first is from the Environment Agency (EA), with a classic approach of integrated pollution prevention and control (IPPC) to odour management.
The implications of the EA's IPPC H4 Horizontal Guidance note on the water industry are not yet fully understood. A limited number of sludge and solid waste processing facilities will fall under IPPC control, those facilities that do fall under IPPC odour regulation will need to secure permits with associated emission limits. By October 2007 the facilities will need to meet these stringent emission standards. Investment required will need to be consistent with best available techniques (BAT), a current UKWIR project with contributions from water utilities is tasked to establish an IPPC guidance note for sludge treatment sites within which BAT for odour control will be defined.
A draft code of practice (COP) has been published by Defra, the closing date for comments was the end of March 2005, comments were made both by Water UK and individual water undertakers including Thames Water. The COP focuses on a phased approach to solutions, after an initial investigation. As it stands the COP references a best practicable means (BPM) guidance document which is not yet published, a BPM guide is being developed as a consultative process between Defra and the water undertakers and is due to published by July 2005.Odour minimisation
Public acceptance of a new sewage works depends on how well odours are managed. Therefore, a full design, looking at odour and airflow minimisation, containment, treatment and dispersion needs to be carefully considered. The aim is to 'design out' any odour production from the start, rather than 'bolting on' an odour solution after the fact.
Acknowledging there will be emissions from bioreactors, the entire activated sludge plant can be covered. Headspace of unaerated fractions can be ventilated to the main process air blowers. This foul air stream, diluted by the major flow of process air, will receive biological treatment as it passes through the aerated fraction as fine bubble diffused air. The aerated lanes' headspace is then extracted with fans that step up and down with the blowers, and the air is dispersed via a high-level stack.
Process selection for both the sludge and effluent streams can also be influenced by the need to minimise odour at source and, where necessary, to secure containment and extraction. The two main characteristics in any plant selected should be their ability to accept odour control covers and, once covered, to offer an acceptable level of operability and maintainability. Ideally, all serviceable components should be located outside the odour control cover. Typical WwTWs produce three types of materials that must be taken off-site for reuse or disposal - screenings, grit and biosolids. Screenings and grit must comply with the requirements of the Landfill Directive.
The grit and screenings are cleaned, dewatered and stored in an odour controlled environment, while still allowing for skip movement and access for quality monitoring. To maintain the practice of recycling treated sludge to agriculture, the product must be free of Salmonella and have been treated to destroy 99.9999% of the pathogens.
The Safe Sludge Matrix refers to this as being an 'enhanced treated product, which is less likely to cause odours when stockpiled prior to being spread on agricultural land. Odour modelling shows that, irrespective of location within a site, any solids storage pad needs containment and odour treatment, which can be achieved by a cake silo storage. The high aspect ratio of solids silos reduces the requirement for ventilation and odour treatment.
At many UK works, up to 40% of H2S emissions are generated by poor design or by poor process control of the sludge stream. This can be prevented by a tight control of the time it takes for the sludge to pass from the primary tanks and the bioreactor to the storage silos. All solids handling equipment, including the gravity sludge thickening belts and dewatering centrifuges, need to have a track record in odour containment.Odour containment
Despite best efforts to minimise odours, remaining emissions must be contained and not be released without treatment. Design can be based on odour generation rates observed at sites. Building an odour mass and flow balance model helps to select the optimal odour management solution.
All unit processes with high emissions, such as in the inlet, sludge and dewatering buildings should be under local control. The building envelope, also odour controlled, acts as a second level of containment. Ideally, storm tanks are covered, cleaned once empty and odour controlled.
An exception can be in primary settling when lamella separators are used, and covers would hinder operation's ability to maintain and operate the settlers. Lamella tanks are therefore better not covered, but enclosed in a building with extractions to the odour control plant at six air changes per hour. Dispersion modelling confirms air collected from the unit processes and buildings, as well as exhaust from appurtenances burning biogas, needs to be treated before entering the atmosphere to avoid causing an odour nuisance. An abatement solution that provides a best whole-life cost consists of:
Other options include biofilters and further use of the bioreactor but are often rejected because of either land take, ducting or risk. For example, a low technology biofilter along the site perimeter can be rejected because of the amount of ducting required.Health and safety
If buildings are used for containment, a traffic light system linked to the status of the ventilation system and air monitors will warn operators if entry is at risk. Undetected hydrocarbon spills that enter the works pose risks to a covered works. For this reason, hydrocarbon monitors need to be installed in the feed pump station's wet wells, within the inlet works odour ducting, in the inlet building drainage sump and at the bioreactor inlet.
On detection of a spill, the operator can use a rapid acting penstock to divert contaminated flow to the blind storm tanks. To ensure operations can gain access to pipework to clear blockages caused by struvite or carbonate scale, the digester and sludge storage tank and the joining pipework need to be extracted to the odour control system.
The ventilation rate needs to be maintained high enough to dilute the methane gas concentration to 20% of its lower explosion limit. The successful operation of an odourless plant requires diligence and an enhanced level of expertise with respect to the technologies employed. A comprehensive package of online sensors can monitor the performance of the abatement plant. Auditable records will be secure through the site-wide Scada system.
H2S is an appropriate marker gas so online stack measurement of H2S can indicate performance and quickly highlight any deterioration in abatement. Key mechanical plant items will be monitored, with alarms routed to a manned control and co-ordination centre.
In the new Reading WwTW, the design process has recognised odour as one of the key drivers from concept through to operation. Therefore, a site-wide approach was taken beyond classical sewage and sludge septicity, encompassing both the main process streams and residuals such as grit and screenings.
The mass balance for the ventilation design has established a hierarchy in odour sources to minimise generation by design. This strategy, supported by local containment and ventilation, has enhanced the working environment and promoted the use of architecturally appropriate structures.