Preventing Odor Emissions at WWTPs by Integrated Approaches

The general approach to reduce odor emissions is the implementation of odor abatement technologies, which entails important costs and often requires compelling operator efforts. This end-of-pipe treatment approach addresses odor nuisance management once odorants have been produced and released from the wastewater. In this context, a more desirable approach would be the prevention of odorant formation and/or release from the wastewater. Limited options are available for the prevention of odorant release at WWTPs beyond proper design and good operating practices such as maintaining aerobic or anoxic conditions in the wastewater where possible, frequent cleaning of process units, minimization of the sludge retention time in thickeners and dewatering systems or the use of buildings and covers to confine the emission in key operation units. So, the general approach of implementing odor abatement technologies to resolve odor problems often require an expensive upgrading of the plant and increase operating costs, while having a limited potential to control the odor generation.

Two widely-applicable, emerging odor control technologies known as Activated Sludge Recycling (ASR) and Oxidized Ammonium Recycling (OAR) possess a significant odor prevention potential for WWTPs at low investment and operating costs. Despite these technologies have been discussed in technical forums and applied at some full scale in WWTPs with promising results, their fundamentals and optimal conditions for odor prevention have not been explored using a systematic approach.

A review recently published in Bioresource Technology presents and critically discusses the fundamentals and optimal conditions of ASR and OAR for odor control based on all technical information available to date. This aim was to provide a stepping stone for a more widespread application or at least have these technologies available as a tool to be considered when developing plant wide odor management strategies. Application of this compiled knowledge demonstrated convincing results in odor reduction performance and confirmed the positive contributions of ASR and OAR to the sustainability and economic efficiency of odor control at WWTPs.

“The ASR strategy promotes the consumption of odorous compounds before they volatilize from the liquid phase”

Activated sludge recycling

Activated Sludge Recycling (ASR) is a strategy for odor control consisting of the recycling of waste or return settled activated sludge from secondary clarifiers or aerobic activated sludge from aerated biological reactors to the inlet of the WWTP headworks (Figure 1). The implementation of ASR would require relatively low investment costs, accounting for the pipeline to transfer the sludge to the headworks and the pumping equipment needed. The additional operating costs would derive from the power needed for pumping and the maintenance of this equipment. The requirement for covering of process units, foul air extraction ductworks, blowers or odor treatment unit could be eliminated. The ASR strategy promotes the consumption of odorous compounds before they volatilize from the liquid phase. Adsorption to the activated sludge flocs followed by oxidation of potential malodorous compounds is assumed to be the mechanism preventing their release from the subsequent wastewater treatment units.

The recycled activated sludge from the aeration basin or the secondary settler contains significant concentration of oxygen (typically 2-3 mg/l) and/or nitrate (typically 6-10 mg/l) that will be used as electron acceptors for the oxidation of the odorants or the malodorous compound precursors. Biological odorant oxidation can thus take place by aerobic oxidation or anoxic oxidation coupled to denitrification. When oxygen or nitrate availability is limited, the production and precipitation of elemental sulfur is likely to occur.

Activated sludge usually exhibits a high biological diversity holding the potential to adsorb and biologically oxidize most biogenic compounds responsible for odor nuisance (mainly reduced volatile organic or inorganic compounds such as H2S, mercaptans, amines, indoles and fatty acids). In fact, the diffusion of malodorous emissions into aeration basins, known as activated sludge diffusion (ASD), has been employed as a method for odor control for more than 30 years, and activated sludge is commonly employed as inoculum for standard biological odor treatment systems such as biofilters and biotrickling filters.

The effect of iron salts, often added during wastewater treatment for phosphorus precipitation, present in the recycled sludge liquor can be also beneficial for odor prevention by promoting the precipitation of dissolved sulfide as ferrous sulfide. ASR can reduce the release of odorous compounds from the wastewater in the inlet works, pre-treatment, pumping stations and primary settlers, which are usually reported as the main responsible process units for malodorous emissions at WWTPs.

Typical WWTP flow diagram including two different options for ASR operation with 1. direct ASR from the aerobic activated sludge reactor (dotted line) and 2. ASR from the secondary settler (dashed line).

The risks of creating a negative influence on the biological wastewater treatment process or activated sludge floc sedimentation properties (about abundance in sulfur consuming and filamentous bacteria) are small or are good possible to control. One study revealed changes in the grit settling properties due to the mixture of the raw wastewater and recycled activated sludge, which might affect the grit removal process. The addition of large amounts of RAS to the raw sewage can create a “fluffier”, misshapen grit that settles slower than what would normally be expected for particles of similar size in pure raw sewage. As a result of these findings, a design was developed and successfully implemented that limits the amount of RAS to be mixed with the raw sewage to provide the desired amount of odor treatment without impacting the grit removal process.

The technology can be applied for a wide range of odor loads into the wastewater treatment plant, which typical has an average sulfide concentrations in the wastewater of about 2 to 6 mg/l. Pilot tests and full-scale applications have shown that long-term consistent H2S removal efficiencies of 90 to 95 percent can be easily achieved when the technology is properly implemented.

“Pilot tests and full-scale applications have shown that long-term consistent H2S removal efficiencies of 90 to 95 percent can be easily achieved when the technology is properly implemented”

Oxidized ammonium recycling

A typical nitrogen removal wastewater treatment plant consists of an aerobic section in the biological reactor in which ammonia nitrogen is oxidized by nitrifying bacteria to nitrate and nitrite. In the anoxic section, denitrification takes place reducing nitrate and nitrite to nitrogen gas using organic matter as electron donor. There are several configurations for nitrification-denitrification reactors, and despite the order of the stages can be inverted, their fundamentals remain similar.

Oxidized Ammonium Recycling (OAR) is based on the recycling of streams with high nitrate or nitrite concentration to the inlet works of a WWTP or upstream in the sewer system (Figure 2). This strategy is commonly implemented in existing denitrification-nitrification plants to reduce nitrogen levels discharged to receiving water bodies in order to meet discharge limits after changes in regulation or plant expansion/upgrading to include anaerobic digestion. Effluents with high NH4+ concentration are nitrified and recycled to the inlet works where they undergo denitrification. However, there are several experiences reporting significant odor reductions as a side-effect of the implementation of OAR. The addition of nitrate to the wastewater influent promotes anoxic conditions, where nitrate is used as an electron acceptor by microorganisms in order to oxidize dissolved sulfides and any readily biodegradable odorants, preventing their further release as malodorous emissions.

WWTP flow diagram including the sludge line and two different options for NR operation. 1, dotted line: NR from centrate nitrification units. 2, dashed line: NR from the nitrification stage in the biological reactor.

The quest for energy and economic efficiency in WWTPs has made anaerobic digestion a widespread technology for sludge management in order to generate part of the electricity consumed during wastewater treatment. However, the de-watering of the anaerobically digested sludge generates ammonia rich effluents (500-1000 mg/l) commonly referred to as centrates or reject waters. Traditionally, this ammonia-rich effluent, representing up to 20 percent of the total ammonia load to the WWTP, is recycled to the biological treatment to be removed by conventional nitrification-denitrification process, increasing the overall wastewater treatment costs and challenging the compliance with the nitrogen discharge limits. Thus, innovative treatment technologies for these centrate streams have been developed, some of them including the biological oxidation of ammonia to nitrate and its further recycling to the WWTP Inlet Works.

In addition, other sources of nitrate-rich streams such as nitrified wastewater from the nitrification stage have been explored in order to achieve effective odor reduction in the receiving wastewater (Figure 2). When sludge is not recycled together with the nitrate-rich effluent, the process will rely on the indigenous biodiversity present in the sewage to perform the anoxic oxidation of the malodorous compounds. All previously mentioned advantages for ASR would also apply to the OAR strategy, including the low investment and operating costs and ease of operation.

“The implementation of ASR and OAR strategies holds the potential to prevent malodorous emissions from WWTP at low investment and operating costs”


The implementation of Activated Sludge Recyling (ASR) and Oxidized Ammonium Recycling (OAR) strategies holds the potential to prevent malodorous emissions from WWTP at low investment and operating costs. Their simple operation, and the use of streams readily available in any WWTP, make them an economical and sustainable method to be considered when developing plant wide odor management strategies. Operational issues related to the hydraulic WWTP capacity, the potential deterioration of the sludge settling properties and the potential incompatibility with further biological processes (e.g. phosphorous removal) have to be considered during the implementation phase. Properly design implementations of ASR and OAR demonstrated convincing performance results in odor reduction and confirmed their positive contribution to the sustainability and economic efficiency of WWTPs.

Bart Kraakman is principal process engineer and a regional odor and air quality lead for CH2M, an employee-owned global leader in full-service consulting, design, design-build, operations and program management services for public and private clients. CH2M received the 2012 Global Water Award for Water Company of the Year and the 2015 Stockholm Industry Water Award.

Bart Kraakman
Principal Process Engineer