wastewater treatment plant photo
By Bart Kraakman

The general approach to reduce odour emissions is the implementation of odour abatement technologies. This end-of-pipe treatment approach addresses odour nuisance management once odorants have been produced and released from wastewater treatment. 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 wastewater treatment plants (WWTP) 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 emissions to key operation units.

Implementing abatement technologies to resolve odour problems often requires an expensive upgrading of the plant and increased operating costs, while having limited potential to control odour generation.

Two widely-applicable, emerging odour control technologies, known as activated sludge recycling (ASR) and oxidized ammonium recycling (OAR), have significant potential for WWTPs with low investment and operating costs. Although these technologies have been discussed in technical forums and applied at some WWTPs with promising results, they have not been explored using a systematic approach.

A recent review by Estrada et al. (2015a and 2015b) presents and critically discusses the fundamentals and optimal conditions of ASR and OAR for odour control, based on all technical information available to date. Their aim was to provide a stepping stone for more widespread application, or at least to have these technologies available as a tool to be considered when developing plant-wide odour management strategies.

Convincing results were demonstrated for sustainable and economical odour reduction performance at wastewater treatment plants.

Activated sludge recycling

ASR means 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. Implementing it requires a pipeline and pumping equipment to transfer sludge to the headworks. (See Figure 1)

wastewater treatment plant flow diagram
Figure 1. 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).

There are some additional operating costs for the power needed for pumping and the maintenance of equipment. However, the need for covering process units, foul air extraction ductworks, blowers or odour treatment unit could be eliminated.

The ASR strategy promotes 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.

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

Activated sludge usually exhibits high biological diversity, with the potential to adsorb and biologically oxidize most biogenic compounds responsible for odour 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, has been employed as a method for odour control for more than 30 years. Activated sludge is commonly employed as inoculum for standard biological odour treatment systems, such as biofilters and biotrickling filters.

Iron salts are often added during wastewater treatment for phosphorus precipitation, and their presence in the recycled sludge liquor can also be beneficial for odour 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, pretreatment, pumping stations and primary settlers. These are usually reported as the main process units responsible for malodorous emissions at WWTPs.

The risk of negatively affecting the wastewater treatment process or activated sludge floc sedimentation, is small and easy to control. One study revealed changes in 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 return actived sludge (RAS) to the raw sewage can create a “fluffier”, misshapen grit that settles more slowly than what would normally be expected for particles of similar size.

As a result of these findings, a design was developed and successfully implemented that limits the amount of RAS to be mixed with raw sewage, to provide the desired amount of odour treatment without impacting the grit removal process.

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

Oxidized ammonium recycling

Typical nitrogen removal at a wastewater treatment plant consists of an aerobic section in the biological reactor. Here, 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.

With OAR, streams with high nitrate or nitrite concentrations are recycled to the inlet works of a WWTP, or upstream in the sewer system. 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+ concentrations are nitrified and recycled to the inlet works where they undergo denitrification. However, there are several reports of significant odour reductions as a side effect of the implementation of OAR. The addition of nitrate to the wastewater influent promotes anoxic conditions. It 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.

The quest for energy and economic efficiency in WWTPs has made anaerobic digestion a widespread technology for sludge management in order to generate electricity for the plant. However, dewatering of the anaerobically digested sludge generates ammonia rich effluents (500 mg/l – 1000 mg/l). Traditionally, this ammonia-rich effluent, representing up to 20% of the total ammonia load to the WWTP, is recycled, to be removed by the conventional nitrification-denitrification process.

This increases overall wastewater treatment costs and challenges compliance with nitrogen discharge limits. Thus, innovative treatment technologies for these centrate streams have been developed. These include 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 odour reduction in the receiving wastewater (See Figure 2).

wastewater treatment plant flow diagram showing sludge line
Figure 2. 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.

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 anoxic oxidation of the malodorous compounds.

All previously mentioned advantages for ASR would also apply to the OAR strategy, including low investment and operating costs, and ease of operation.


Implementing ASR and OAR strategies holds the potential to prevent malodorous emissions from WWTPs 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 odour management strategies. Operational issues related to the hydraulic WWTP capacity, the potential deterioration of the sludge settling properties and potential incompatibility with further biological processes (e.g., phosphorus removal) have to be considered. Proper design and implementation of activated sludge recycling and oxidized ammonium recycling has demonstrated convincing performance results in odour reduction.

Bart Kraakman is with CH2M. This article appears in ES&E Magazine’s April 2017 issue.



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