Dewatering Sidestream Treatment: Increased Nutrient Recovery and Reduced Liquid Treatment Impacts

Authors:

  • Katya Bilyk, Ron Taylor, Paul Pitt, Theresa Bruton, Laurissa Cubbage, Leah Flowers, and Phill Yi - Hazen and Sawyer

Figure 1. Biological Nitrogen Removal Cycle and Deammonification Pathway

A sidestream is any process flow resulting from the treatment of biosolids that flows back into the liquid treatment train. Examples include but are not limited to: belt filter press (BFP) filtrate, gravity belt thickener (GBT) filtrate, filter backwash, centrate, and digester supernatant. Sidestream treatment therefore, refers to the interception and manipulation of the sidestream with a treatment removal objective. Typically, the treatment objective is to remove nutrients.

When primary sludge and/or waste activated sludge is stored for any significant period (i.e., digested), additional biological reactions occur that increase the concentration of dissolved nutrients in the wastewater. These dissolved nutrients concentrate during solids separation steps such as thickening and dewatering, resulting in high concentrations of dissolved nutrients in the sidestream.

Sidestream treatment is often economical due to the relatively low volume and high concentration of nutrients present. For instance, sidestream nitrogen and phosphorus loads from wastewater treatment plants that perform biological nutrient removal (BNR) and have anaerobic digestion typically account for 15-20 percent of influent nitrogen, and 20-30 percent of influent phosphorus. The flow is about one percent of the forward flow.

Sidestream treatment also has a small carbon footprint, and provides a link between today’s wastewater treatment plants and wastewater treatment plants of the future, which will mine nutrients from these sidestreams.

There are many benefits to sidestream treatment, such as increasing the factor of safety on nitrification and biological phosphorus removal, which in turn allows lower nutrient standards to be achieved with conventional technology. The nutrient peaking factor to the BNR facility is also reduced, which results in more stable operation.

Purpose

The purpose of this paper is to (1) present the spectrum of sidestream treatment alternatives for nitrogen and phosphorus removal, (2) discuss which approaches are most economically and functionally viable, and (3) present the results of recent sidestreatm treatment evaluations to show their practical benefit.

Results

Sidestream nitrogen removal is primarily accomplished biologically using one of three approaches: bioagumentation, denitritiation/nitritation, and nitritation/deammonification. Of these approaches, nitritation/deammonification is most efficient. This process shortcuts the traditional nitrification process by preventing nitrite oxidizing bacteria (NOB) from growing through strict solids inventory control. By doing so, nitrite is not oxidized to nitrate, and aeration costs are reduced by 62.5%.

Deammonification is performed by slow growing autotrophic bacteria that utilize ammonia as an electron donor, which offers a significant operating cost savings as well. The traditional nitrification and denitrification cycle is shown above, along with the nitritation/deammonification shortcut. In a recent evaluation, the net present worth cost savings for nitritation/deammonification were 50 to 75 percent less than other biological nitrogen removal side stream treatment processes, and the payback period was less than five years.

Another promising and efficient sidestream treatment alternative is the Ostara® process. Ostara is a centrate treatment process aimed at simultaneously recovering both phosphorus and nitrogen by adding magnesium chloride and sodium hydroxide to precipitate out struvite pellets (MgNH4PO4). The pellets are a sustainable end-product that can be marketed as a slow-release fertilizer. Several recent cost evaluations have demonstrated that the Ostara process is less expensive than chemical precipitation with coagulants on a net present cost basis.

Another complimentary add-on concept to the struvite precipitation process is the patented WASSTRIP® process. This process aims to significantly improve struvite formation potential at treatment plants by isolating and removing significant amounts of two of the three constituents that form struvite (magnesium and orthophosphate) at the beginning of the solids handling process. Therefore, struvite formation potential is significantly reduced until the two sidestreams (thickening filtrate and dewatering filtrate) are intentionally combined in a struvite precipitation reactor.

To learn more about related work, visit this project page.

To request a copy of the full paper, please contact the author at kbilyk@hazenandsawyer.com.

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