Lessons Learned from the Startup of Sidestream Deammonification Facilities
- Joe Rohrbacher, Katya Bilyk - Hazen and Sawyer
Solids processing operations at wastewater treatment plants often produce sidestreams from thickening, decanting and dewatering. These sidestream flows can contribute significant soluble nitrogen and phosphorus loads back to the liquid stream process, particularly if digestion processes are employed. 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 even though the sidestream flow may only account for 1% of the total plant flow. Since dewatering is typically a batch operation, the impacts of sidestream flows can be more pronounced and cause upsets in the secondary process, increasing effluent nutrient concentrations. Nutrient loads from sidestreams can make compliance with effluent nutrient limits significantly more difficult and costly.
Capturing and treating solids sidestream flows (sidestream treatment) before reintroduction to the influent is often economical because of the high concentration of nutrients, warm temperature, and small volume that needs to be treated. 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.
Recent innovations in sidestream nitrogen treatment have reduced the cost of removing nitrogen from solids recycle flows. Sidestream deammonification utilizes Anammox bacteria to convert ammonia directly to nitrogen gas in the presence of nitrite. This pathway reduces oxygen requirements by approximately 63% compared to the conventional nitrification-denitrification process employed in BNR systems, and can reduce or eliminate the need to feed supplemental carbon to meet low total nitrogen (TN) limits. Multiple studies have shown the cost per pound of nitrogen removed is on the order of $0.50-$0.75 per pound of TN removed using deammonification compared to $1-$3 per lb of TN removed using conventional BNR. Payback on capital investments tends to vary between 8 and 12 years.
This presentation will present a variety of lessons learned about sidestream deammonification as a result of multiple studies, pilot work, and full-scale experience. Some of the key lessons learned from startup and operation of deammonification systems are as follows:
1. Small variances in process controls can lead to different microbial communities and differences in nitrogen removal; therefore for future designs a maximum of two reactors are recommended to minimize operational complexity.
2. In fixed-film systems, measuring TSS in the reactors can serve as a proxy for nitrite oxidizing bacteria (NOB) growth potential and can be used to monitor NOB growth. Dilution water can also be used to decrease detention time and wash out suspended NOB growth.
3. Insulation, tank covers and/or ancillary heating may be required to maintain a minimum sidestream temperature of 25°C to optimize the deammonification process.
4. With the incorporation of thermal hydrolysis processes (THP) and other pre-digestion treatment, the nitrogen load contributed by sidestreams may increase by an additional 20 percent. Special consideration must be given when evaluating deammonification for systems that pre-treat solids prior to digestion.
5. Supplemental phosphorus may need to be fed if insufficient phosphate is available in the sidestream.
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