Understanding the Impact of Refractory Thermal Hydrolysis Products on ENR WWRFs
- W. O. Khunjar, R. Bunce, R. Latimer, K. Bilyk, R. Taylor, M. Bullard, A. Stone, P. Pitt - Hazen and Sawyer
Transitioning toward energy neutral or positive operation at water resource reclamation facilities (WRRF) requires an integrated approach whereby energy recovery is maximized while energy consumption is reduced (WERF, 2011a). Presently, energy recovery at WRRFs is primarily achieved via methane recovery from anaerobic stabilization processes; however, methane production from anaerobic digestion can be limited by the rate of solids hydrolysis during digestion (Pavlostathis, 1986).
Thermal hydrolysis of municipal solids represents an opportunity for WRRFs to increase the rate of solid hydrolysis, increase the extent of volatile solids reduction and boost biogas production during anaerobic digestion. Alongside this increased rate and extent of solids hydrolysis is a concomitant increase in nitrogen (N) and phosphorus (P) solubilization. A subset of this released nutrient pool is present as melanoidins, which are nitrogenous organics (Dwyer et al., 192008). These melanoidins, formed via the Maillard reaction, are recalcitrant to biodegradation during anaerobic or aerobic processes (Dwyer et al., 2008) and are present in dewatering sidestreams.
Typically, WRRFs that are required to meet enhanced nutrient removal (ENR) limits; i.e., TN < 3 mg/L and TP < 0.18 mg/L, utilize some form of biological and/or chemical sidestream treatment to reduce the bulk nutrient loads prior to return of the flow to the mainstream biological process. These sidestream treatment processes have little to no impact on the removal of refractory nitrogen and phosphorus thermal hydrolysis products. Therefore these refractory products are recycled to the mainstream process and increase the total effluent nitrogen and phosphorus concentrations.
For ENR WRRFs, this increased load of refractory thermal hydrolysis nutrient products in the recycle stream can induce a significant operating burden as the facility will need to either remove the refractory products or purchase additional chemicals to remove more readily accessible nutrient fractions (e.g., nitrate and/or nitrite and ortho-phosphorus). To date, the vast majority of studies associated with thermal hydrolysis have focused on quantifying energy recovery potential while few studies have explored the whole plant tradeoffs associated with instituting thermal hydrolysis for increasing volatile solids reduction and increasing biogas production versus the increased degree of sidestream or mainstream nutrient removal treatment required at ENR facilities.
In this paper, we will present results from a holistic evaluation of thermal hydrolysis, energy recovery and nutrient removal requirements at an ENR facility in the Mid-Atlantic region. As part of this work, we will quantify the true operational costs associated with achieving ENR standards while performing thermal hydrolysis.
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