Odour control is an increasing concern in WWTPs. Physical/chemical end-of-the-pipe technologies for odour abatement are relatively expensive and present high environmental impacts. Biotechnologies, on the other hand, have recently emerged as cost-effective and environmentally friendly alternatives but are still limited by their investment costs and land requirements. A more desirable approach to odour control is the prevention of odorant formation.
In WWTPs, where different biological processes take place and many streams are available, there are opportunities to re-design processes in order to minimize odour generation. This work explores two alternative strategies for odour control.
J. M. Estrada 1,4, R. Lebrero 1, N. J. R. Kraakman 2,3 and R. Muñoz 1*
1. Department of Chemical Engineering and Environmental Technology, University of Valladolid. Dr. Mergelina s/n, 47011, Valladolid, Spain.
2. Department of Biotechnology, Delft University of Technology, Julianalaan 67, 2628 BC Delft, The Netherlands.
3. CH2M Hill, Level 7, 9 Help Street, Chatswood NSW 2067, Australia.
4. Present address: School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, United Kingdom.
Competing interests: The author has declared that no competing interests exist.
Academic editor: Carlos N Díaz.
Content quality: This paper has been peer reviewed by at least two reviewers. See scientific committee here
Citation: J. M. Estrada, R. Lebrero, N. J. R. Kraakman and R. Muñoz, 2015, Activated sludge recycling and oxidized ammonium recycling: innovative strategies for odour prevention in WWTPs, III International Conference of Odours in the Environment, Bilbao, Spain, www.olores.org.
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Keyword: WWTP, H2S, activated sludge, nitrates, odours.
Odour control is an increasing concern in WWTPs. Physical/chemical end-of-the-pipe technologies for odour abatement are relatively expensive and present high environmental impacts. Biotechnologies, on the other hand, have recently emerged as cost-effective and environmentally friendly alternatives but are still limited by their investment costs and land requirements. A more desirable approach to odour control is the prevention of odorant formation before being released to the atmosphere, but limited advances have been made in this field so far. In WWTPs, where different biological processes take place and many streams are available, there are opportunities to re-design processes in order to minimize odour generation. This work explores two alternative strategies for odour control, Activated Sludge Recycling (ASR) and Oxidized Ammonium Recycling (OAR), by discussing their fundamentals, operating parameters and the available experience. Both technologies are widely applicable and cost-efficient, making use of readily available plant by-products. Results from full scale facilities show that the application of ASR and OAR strategies can reduce H2S and mercaptans concentration by a 60-98% in malodorous streams at certain plant locations. Odour reductions of 50-89% have been reported in ASR scenarios (measured as odour threshold reduction and Standard Odour Emission Rates). Calculations show their low CO2 footprints, and even if odours are not completely eliminated these strategies allow a significant reduction in the need and cost of further odour treatment. ASR and OAR are expected to become common operating strategies in a near future, contributing to the sustainability and economic efficiency of odour control in WWTPs worldwide.
Odour management has become a priority in the design and operation of WWTPs due to the enforcement of new environmental regulations and to the increasing awareness of companies operating WWTPs about their public image, (Easter et al., 2008; Estrada et al., 2011). Impact minimization (via passive barriers installation or chemicals spraying) and end-of-pipe strategies address odour 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: 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. Unfortunately, many of these solutions often require an expensive plant upgrading or re-design, increase process operating costs and have a limited potential to control the odour generation.
The present paper reviews two widely-applicable, emerging odour control technologies known as Activated Sludge Recycling (ASR) and Oxidized Ammonium Recycling (OAR), which possess a significant odour prevention potential for WWTPs at low investment and operating costs (Estrada et al., 2015; Husband et al., 2010; Kiesewetter et al., 2012).
2. Materials and Methods
This work collects information on ASR and OAR strategies from technical forums, grey literature and the available experience of their application in full scale WWTPs. Despite the promising results over the past decade, their fundamentals, limitations and potential for odour prevention have not been fully explored yet.
3. Results and Discussion
3.1 Activated Sludge Recycling
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).
ASR consists 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). This allows the consumption of malodorous compounds prior to their volatilization from the liquid phase. Adsorption followed by oxidation of potential malodorous compounds is assumed to be the mechanism preventing their release in the subsequent wastewater treatment units (Kiesewetter et al., 2012). Sulphide concentration in the raw wastewater is the key parameter to determine if an ASR strategy for reducing odour problems in a WWTP is suitable and to define the Sludge-to-Wastewater ratio to be recycled, since the rest of the parameters are often fixed by the efficient operation of the biological wastewater treatment and usually remain stable over time. As a rule of thumb, it seems reasonable to assume that a 0.5 activated sludge recycling ratio will achieve H2S gas concentration reductions ranging from 50 to 75% when dealing with average sulphide concentrations in the wastewater of ≈5 mg L-1. Full scale implementation of ASR in WWTPS located in Florida and Texas reveal reductions of 60-98% of H2S concentration in malodorous streams and total odour reductions up to 89% (Houston, 1994; Harrison et al., 2008; Schmidt et al., 2014; Kiesewetter et al., 2012; Koetter et al., 2014).
In terms of process economy, the implementation of ASR would require very low investment costs, accounting for the pipeline to transfer the sludge to the headworks and the pumping equipment needed. Any additional operating costs would only derive from the power needed for pumping and the maintenance of this equipment. The requirement for covering of process units, foul air extraction ductworks or blowers could be eliminated due to reduced odour generation from these sources, reducing the overall costs when compared to other conventional odour abatement techniques. Preliminary estimations have shown that an ASR system able to reduce the H2S concentration by 80% (from 104.5 to 20.9 ppm) in a malodorous stream would be able to cut the operating costs of an activated carbon adsorption unit by 46% and those of a chemical scrubber by 74% (Estrada et al., 2011).
3.2 Oxidized Ammonia Recycling
Figure 2. WWTP flow diagram including 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.
OAR strategy recycles streams with high nitrate or nitrite concentration (300-1000 mg L-1) to the inlet works of a WWTP or upstream in the sewer system. This strategy is commonly implemented to reduce nitrogen levels discharged to receiving water bodies in denitrification-nitrification plants (Constantine, 2006). However, significant odour reductions have been reported as a side-effect of the implementation of OAR, and nowadays it is considered also as an odour reducing technique (Bratby et al., 2011; Husband et al., 2010). Adding nitrates to the wastewater influent creates anoxic conditions, where nitrate is used as an electron acceptor by microorganisms to oxidize dissolved sulphides and readily biodegradable odorants, preventing their further release as malodorous emissions. Experiences carried out in WWTPs in Arizona, Colorado and California report H2S gas concentration reductions of 60-75% after the implementation of OAR strategies (Husband et al., 2010; Bratby et al., 2011; CH2M HILL, 2014).
Nitrate concentration in the nitrified stream recycled will be the key parameter to provide enough sulphide oxidation potential. Assuming a conservative concentration of nitrate in the stream after the nitrification of an ammonia-rich centrate (500 mg NO3- L-1), only a 0.2% recycling ratio would be needed to allow sulphide oxidation in a typical wastewater (e.g. 60 m3 day-1 for a raw wastewater influent of 30,000 m3 day-1, containing 0.5-10 mg L-1 of sulphide). However, contrary to ASR, OAR does not provide the biological catalyst required to perform S2- oxidation, which might lead to biological limitations of this strategy. New hybrid ASR-OAR strategies would provide high electron acceptor concentration in nitrified streams combined with the biological activity of activated sludge hold a strong future potential and deserve further research.
3.3 Environmental and economic considerations
Despite only preliminary calculations have been carried out, previous works have shown how the ASR strategy presents a minimum carbon footprint. The energy consumption of this technology would only come from the recycled stream pumping, being similar to the energy consumption of a biotrickling filter. However, recycling strategies present no need to manufacture packing materials and minimize the investment and installation costs. (Estrada et al. 2015). As an example of the achievable benefits, the operating costs associated to the post-treatment of this malodorous emission by means of a two-stage chemical scrubber were reduced by 59% by the installation of an OAR system in a WWTP in Phoenix, Arizona (from 460 to 150 USD per day). In addition, investment costs savings were estimated in 40 million USD by avoiding the need to cover the 12 existing primary settlers and the treatment of the foul air in chemical scrubbers (Husband et al., 2010).
The implementation of ASR and OAR strategies holds the potential to prevent malodorous emissions from WWTP at low investment and operating costs. Operational issues related to the hydraulic WWTP capacity, the potential deterioration of the sludge settling properties and the eventual incompatibility of ASR and OAR strategies with further biological processes (e.g. phosphorous removal) should be carefully addressed in future research.
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