The Potential for Co-Digestion of Organics Using Thermal Hydrolysis at Blue Plains Advanced Wastewater Treatment Works

THE POTENTIAL FOR CO-DIGESTION OF ORGANICS USING THERMAL HYDROLYSIS AT BLUE PLAINS ADVANCED WASTEWATER TREATMENT WORKS Bill Barber1, Chris Peot2, Matt Higgins3, Bo Bodniewicz1, Jim Marx1, Walter Bailey2, Bernhard Wett, Saul Kinter2, Ahmed Al-Omari2, Sudhir Murthy2 1. AECOM, 2. DC Water, 3. Bucknell University ABSTRACT The District of Columbia Water and Sewer Authority (DC Water) have recently installed a state of the art biosolids processing facility at its Blue Plains wastewater treatment works. The facility, the first of its kind in North America, will generate approximately 10 MW electricity by thermally hydrolyzing and digesting site-generated sewage sludge. In addition, it will produce Class A biosolids cake which further reduces DC Water’s carbon and environmental footprint. In order to optimize the use of this facility, DC Water are investigating the impact of co-digesting other organic waste materials to further increase renewable energy and provide financial benefits for its rate-payers. Tests looking specifically at food-waste show it to be a good candidate for co- digestion. When compared to two control reactors in a laboratory study, a digester supplemented thermally hydrolyzed food-waste to similarly processed sludge, as an increase in COD loading, resulted in an increase in biogas consistent with the loading rate and improved biodegradability of the food-waste fraction. This increased the total COD destruction and volatile solids destruction (by mass balance) compared to the controls. Due to high degradability, the solids yield produced from the food-waste was much lower than that from sludge. As the food-waste had a higher carbon:nitrogen ratio, its addition resulted in a statistically significant drop in pH which helped reduce free ammonia levels, known to limit the anaerobic digestion process, although not found to be inhibitory in the trials. It was determined that up to 600 m3 of the food- waste could be added daily prior to the need for a major infrastructure upgrade. INTRODUCTION The District of Columbia Water and Sewer Authority (DC Water) operates the Blue Plains Wastewater Treatment Plant (Blue Plains), which is the largest advanced wastewater treatment plant (AWTP) in the country, with a design capacity of 370 million gallons per day (mgd) and a peak capacity of 1.076 billion gallons per day. The plant is located in the District of Columbia between Interstate 295 and the Potomac River, occupying approximately 150 acres. The existing wastewater treatment processes at the Blue Plains AWTP consists of preliminary treatment, secondary treatment, nitrification / denitrification, effluent filtration, chlorination / dechlorination and post-aeration. DC Water has recently undergone a major overhaul of its sludge processing capability. This has resulted in a move away from Class B lime stabilization producing 1,200 wet tons biosolids/day, to the installation of anaerobic digestion preceded by thermal hydrolysis plant – based on Cambi™ technology (Figure 1) – as a conclusion of a previously completed Biosolids Master Plan. The benefits associated with the overhaul are: production of renewable energy in the form of biogas, generation of Class A biosolids and ease of operation. The production of energy, reduction in biosolids production (by approximately 50%), improvement in dewaterability, reduced transport requirements, and the fact that lime is no longer required, all combine to help reduce DC Water’s carbon footprint by 40% and assist in the attainment of energy neutrality. In addition, it is expected that the new infrastructure will reduce operational expenditure by approximately $20 million annually. While the current wastewater configuration at Blue Plains consumes 26 MW of electricity, the new facility is expected to generate approximately 10 MW of power. Based on the expected flows and loads from 330 mgd, energy generated from biogas is expected to be in the order of 0.20 kWhr/m3 influent added. This compares with energy requirements of Blue Plains Advanced Wastewater Treatment works of 0.53 kWhr/m3 published previously. As part of a wider initiative, DC Water is pro-actively pursuing ways to minimize its energy demand and associated environmental impact. This is being achieved by deployment of a multi-faceted approach involving, reduction of aeration demand during nutrient removal by use of deammonification, renewable energy from solar, and anaerobic digestion of sludge produced on site to generate renewable energy through turbines as mentioned above. The production of renewable energy in this way can be further enhanced by exploiting the spare capacity of the anaerobic digestion plant and associated ancillary infrastructure by the addition of external energy sources in the form of waste organics by co-digestion. Figure 1. Section of the Cambi ® thermal hydrolysis process installed at Blue Plains Although co-digestion of waste materials poses challenges, on the whole, it is viewed as highly beneficial, from both a financial and environmental aspect. A recent study by WERF (2010) identified numerous benefits of co-digestion sub-divided into three areas of: technical; economic, and environmental benefits. Local authorities in the US, as well as European Governments see co-digestion as a means to increase production of renewable energy to assist with State targets, reduce reliance on landfill which is not considered sustainable, and reduce carbon impact helping to meet emissions reductions targets. In addition, co-digestion of waste organics at a municipal wastewater treatment works can provide tangible financial benefits which can be used to off-set costs and ultimately passed on to rate payers. A recent study by the EPA (2014) highlighted six success stories where co-digestion has been employed with potential pay-back times from just under 3 to 12 years. The stand-out success story was based on experience at East Bay Municipal Utilities District (EBMUD) which calculates annual benefits of $11M (EPA, 2014), of which over 70% come from tipping fees which range from 3 cents/gallon to $30/ton solid waste. To determine the viability of co-digestion at Blue Plains, a laboratory study was conducted in conjunction with Bucknell University looking specifically at pre-processed food-waste from a Waste Management site in California. The aim of which was to quantify the influence of adding food-waste to thermally hydrolyzed sludge so that the economic and environmental impacts could be determined. Co-digestion of food-waste According to the U.S. Environmental Protection Agency (EPA), food waste represents 14.5% of municipal solid waste (MSW), and most of what’s generated is wasted. Of the 251 million tons of MSW Americans generated in 2012, food waste comprised 36.43 million tons, of which only 1.74 million tons (4.8%) was recovered. An estimated 216 wastewater treatment facilities located in the U.S. haul in food waste (primarily FOG) for co-digestion with sewage sludge. This accounts for approximately 17% of all municipal plants which use anaerobic digestion (EPA, 2014). A great deal of work has been conducted on the anaerobic digestion of food-waste, both as a mono- and co-substrate and with various inocula (Tampio, et al., 2014; Mata-Alvarez, et al., 2014; Spargiminio et al. 2014; Rajagopalan et al., 2014; Kangle et al., 2012, Iacovidou, et al., 2012; Banks, and Zhang, 2010). Table 1 summarizes the main findings. Food-waste co-digestion influences performance of anaerobic digestion in various obvious and not so obvious ways. Being more biodegradable and having inherently higher energy content than sludge, it is clear that addition of food-waste can significantly increase biogas production compared to a case where the food-waste is absent. As food waste has a much higher carbon to nitrogen ratio than sludge, its addition to alter the bulk carbon to nitrogen ratio within a digester has been shown to provide success with improving the digestion process itself (Mata-Alvarez, et al., 2014). However, as food-waste also contains nitrogen compounds which originate from protein, addition of food has been found to increase ammonia levels during anaerobic digestion (Banks and Zhang, 2010). In studying the influence of pre-processed food-waste on standard mesophilic digestion, Kuo- Dahab and co-workers (2014) studied the impact of varying loading rate on digester performance. They added food-waste in proportions of 10, 20, 50 and 100% comparing performance to digesters not fed any food-waste. In terms of loading rates, the addition of 50% food-waste was equivalent to 4.19 kg VS/m3.d. The study concluded that the optimal dosage rate was 50%, however, closer look at the data reveals that improvements in gas production starts to tail-off prior to that addition rate. This may be due to the continued addition of ammonia which may accumulate (Banks and Zhang, 2010) and cause inhibition (Rajagopal et al., 2013) or wash out of trace nutrients required for biogas production (Banks and Zhang, 2010). Table 1. Observations specific to food-waste co-digestion Observation Reference Food-waste is more biodegradable than sludge Rajagopalan et al., 2014; Banks and Zhang, 2010 Food-waste produces a higher gas yield than sludge Banks and Zhang, 2010 Food-waste adversely influences dewatering Fü, et al., 2015 Rajagopalan et al., 2014 Increases sodium and potassium concentration in Rajagopalan et al., 2014 digestate Increases system ammonia Banks and Zhang, 2010; Iacovidou, et al., 2012 Substrate diversification and better nutritional

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