DigestivoreTM: Killing Four Birds with One Stone

A side stream is any process flow resulting from the treatment of biosolids that flows back to the liquid treatment process. Examples of side streams are filtrate or centrate from dewatering operations and supernatant from digestion processes. If a plant recycles very high concentrations of ammonia from side streams, it can be difficult to remove the excess ammonia in the mainstream liquid treatment process making it problematic to comply with effluent discharge limit permit requirements. In fact side stream flows are approximately 1% of the total influent hydraulic plant flow but consequently account for 15% to 40% of the influent total nitrogen load (Pugh and Stinson, 2012). Side stream flows that range from 900 to 1,500 mg/L as nitrogen (N) or more can increase the ammonia concentration in the plant effluent by 3 to 5 mg/L on an average day basis (Phillips, Kobylinski, Barnard, Wallis-Lange, 2006).In addition, side stream flows with excessive ammonia concentrations can create operational issues to liquid treatment process operations, especially to those whose goal is to achieve total nitrogen removal. Excessive ammonia concentration in side streams can cause depletion of carbon to nitrogen ratios since ammonia generally accounts for 60% of the Total Kejldahl Nitrogen (TKN). Reduced carbon to nitrogen ratios can make the denitrification process problematic to achieve. To achieve denitrification when the carbon to nitrogen ratio is depleted in the liquid treatment process adding an external carbon source such as methanol or Micro-C™ may be necessary, which results in increased sludge loads and operating cost.To improve wastewater treatment plant operations and reduce operating cost associated with the addition of chemicals and disposal costs, treatment of side stream flows can be very advantageous. Side stream treatment can theoretically remove about 85% of ammonia, thereby reducing the load that is returned to the head of the plant. Removing ammonia in side streams requires less volume and lower construction costs than if it were treated in the mainstream liquid treatment process. This is due to the concentrated load and higher temperature which leads to faster kinetics in the side stream. Typically side stream treatment management for ammonia is commonly conducted with biological and physical-chemical processes. Notable sustainable biological side stream management processes include nitrification/denitrification, bioaugmentation, nitritatation and denitritation, and deammonification (ANAMMOX). Typical physical-chemical processes used for side stream management include ammonia stripping (steam, hot air, and vacuum distillation), ion exchange (Ammonia Recovery Process (ARP)), and struvite precipitation (Magnesium Ammonia Phosphate (MAP)). Side stream treatment for ammonia utilizing a biological process while integrating waste activated (WAS) thickening with a membrane unit is a new concept.This paper describes the Digestivore™ process, a new development in side stream treatment technology. This process functions as a sludge handling system and also provides biological ammonia treatment of side streams. The Digestivore™ process is a very versatile and robust technology that features an aerobic digestion process utilizing flat plate membranes for thickening waste activated sludge (WAS) and biological treatment of dewatering side streams from solids handling processes that produce high concentrations of ammonia, most notably anaerobic digestion and autothermal thermophilic aerobic digestion (ATAD) systems. This system is suitable for facilities with anaerobic digestion or ATAD systems that must comply with effluent discharge permit limits for ammonia, that have odor problems, and are looking to increase biological capacity but are limited in footprint.