Understanding the Impact of Thermal Hydrolysis Products on ENR WRRFs

Authors:

  • W. O. Khunjar, R. Latimer, K. Bilyk, R. Taylor, M. Bullard, Paul Pitt, M. Mann, P. Stiegel - 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. 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 et al., 1986).

Thermal hydrolysis of municipal wastewater treatment residuals 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., 2008). 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.

For WRRFs required to performed enhanced nutrient removal (i.e., TN < 3 mg/L and TP< 0.18 mg/L), 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 work, we present results from a preliminary evaluation of thermal hydrolysis, nutrient removal requirements and impacts to disinfection at a large ENR facility from the Mid-Atlantic region of the USA.

For more information, contact the author at wkhunjar@hazenandsawyer.com.

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