Comprehensive Biosolids and Bioenergy Planning Authors: Cameron Clark* 1, Irina Lukicheva 1, Anna James 1, Kathy Rosinski 1, Dave Parry 1

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1 Comprehensive Biosolids and Bioenergy Planning Authors: Cameron Clark* 1, Irina Lukicheva 1, Anna James 1, Kathy Rosinski 1, Dave Parry 1 1 CH2M * Cameron.Clark@ch2m.com KEYWORDS: biosolids, bioenergy, market assessment BACKGROUND A comprehensive biosolids and energy plan was prepared for a municipal wastewater treatment plant in California. The wastewater treatment plant (WWTP) provides tertiary treatment for a service area is approximately 143,100 people based on the 2010 census. The WWTP has a permitted capacity of 59.5 million liters per day (ML/d) (15.7 million gallons per day (mgd)) for average dry weather flow (ADWF). The current ADWF is approximately 41.5 ML/d (11 mgd). Anaerobically digested, dewatered biosolids are hauled offsite. The WWTP is currently undergoing upgrades to the liquid treatment train that will improve the effluent water quality and increase the capacity of the plant. Capacity limitation issues, aging infrastructure, and lack of redundancy led the municipality to perform a Biosolids and Energy Management Plan (BEP) to evaluate the solids treatment and bioenergy processes at the WWTP. The BEP evaluated biosolids issues to improve plant operations, such as solids reduction technologies, resource recovery, beneficial use markets, and odors. The municipality defined BEP drivers and their vision of transforming the WWTP into a Water Resource Recovery Facility that will be a model for resource recovery from wastewater and provide cost-effective, diversified, and power-positive energy recovery. The goals of the BEP generally focused on reliability, energy recovery/independence, biosolids end use diversification, and cost effectiveness. The BEP first focused on treatment capacity, redundancy, performance, and condition of the existing biosolids treatment facilities. A market assessment was conducted to review regulatory trends and to identify opportunities for biosolids use, organic waste, and energy markets for biosolids and energy recovery. Market-driven alternatives for biosolids and energy management at the WWTP were then developed and evaluated using a triple bottom line plus multi-attribute model that includes economic, environmental, social, and operations criteria that were developed and prioritized with Copyright 2017 by the Water Environment Federation 1 957

2 input from municipality staff. The preferred alternative was elected based on the cost-to-benefit ratio, which captured both non-monetary criteria and life-cycle costs. Finally an implementation plan was developed for the preferred alternative. The plan was based on preliminary facilities and equipment sizing, estimated project costs, and site layouts. A phasing plan containing a list of projects and a preliminary schedule was developed. EXISTING AND FUTURE CONDITIONS Solids treatment at the plant is shown in Figure 1. It includes primary sludge screening, grit removal, gravity thickening (GT), and waste activated sludge (WAS) thickening in a dissolved air flotation tank (DAFT). Thickened sludge is combined and anaerobically digested in two eggshaped digesters. Digested solids are dewatered by centrifuges to 20 to 23 percent total solids (TS). Class B cake is land applied six months per year during the dry season, and landfilled as alternative daily cover for the remainder of the year. Digester gas is currently used in the boilers to heat water for digester heating and flared. A new compressed natural gas (CNG) facility will use a portion of the digester gas to fuel the City s vehicle fleet. Copyright 2017 by the Water Environment Federation 2 958

3 Biogas Flare WAS DAFT BioCNG Boiler FERRIC Anaerobic Digesters HEAT PS Sludge Screens Sludge Flow Control Chamber Gravity Thickeners BIOGAS Sludge Storage POLYMER Centrifuges Grit Classifier Indicates Pumps Indicates Waste Container Figure 1. WWTP solids treatment process flow diagram Copyright 2017 by the Water Environment Federation 959

4 New preliminary, primary, and secondary treatment facilities are being constructed at the WWTP. These facilities will begin operation in 2023 and are anticipated to result in less grit passing through and accumulating in the biosolids treatment as well as improved sludge thickening. The evaluation identified improvements needed to address capacity limitations, aging equipment, or redundancy issues with the existing biosolids treatment facilities (Table 1). Table 1 Solids treatment processes condition and capacity assessment Unit Process Capacity Limitation? Aged Equipment? Lack of Redundancy? Gravity Thickening NO YES NO Dissolved Air Flotation Thickener NO NO YES Mesophilic Digestion YES NO YES Dewatering Centrifuges YES NO YES Cake Transfer and Storage NO NO YES Boiler NO NO YES Waste Gas Burner NO NO YES The following recommendations were made to improve reliability: Plan for rehabilitation/replacement of existing GT Plan for redundant WAS thickening capacity or implement a redundant workaround Modify digester operation to parallel and add digester capacity Install a third centrifuge (in existing building) Rehabilitate existing cake conveyor or add redundant conveyance capacity Install a second boiler or implement a redundant workaround Copyright 2017 by the Water Environment Federation 4 960

5 MARKET ASSESSMENT A market driven approach to biosolids planning was used to identify the solids processing technologies. The ultimate goal of this approach is to identify biosolids products that can be easily distributed to multiple end uses. Biosolids regulatory update reviewed current and future regulatory trends for management and distribution of biosolids. The biosolids market assessment focused on trends and opportunities for beneficial use. The most common biosolids end use practices are land application of predominantly Class B biosolids during dry season and landfilling or landfill daily cover during the winter months or all year long. Due to the regulatory restrictions currently being placed on disposing of organics in the landfill, the latter option will most likely become limited in the future. This is expected to put pressure on wastewater facilities to find land application sites for year-round land application sites or identify alternative ways to manage biosolids. While land application is likely to continue in the future, more stringent restrictions on Class B biosolids for land application are expected, which will likely increase distribution costs of Class B biosolids. In addition, counties where land application is taking place are increasing the demand for higher quality biosolids, and Class A biosolids are expected to have the most versatile distribution options. The organic waste survey identified opportunities to increase energy recovery from biosolids by augmenting digester feed with local or imported organic wastes. Restrictions on landfilling organics that will limit biosolids landfilling are also expected to increase the amount of food scraps that could be used for co-digestion at wastewater treatment plants. Due to uncertainty of how much organic waste would be available to import to the plant, a moderate co-digestion program was assumed (2-3 trucks per day). The energy assessment identified and described different beneficial uses of renewable energy and energy efficiency. The energy market assessment showed that excess digester gas is expected to be available for beneficial uses, even after the new CNG vehicle fuel production facility achieves full design capacity. Furthermore, there is an opportunity to improve energy recovery with the implementation of a co-digestion program (which would increase digester gas production) and with a combined heat and power (CHP) process (Figure 2). Copyright 2017 by the Water Environment Federation 5 961

6 Figure 2. Projected digester gas production and usage (shown as total heating value of biogas) The projected power demand after secondary treatment upgrades is significantly higher than current levels. To achieve an energy neutral facility using co-digestion, an aggressive (>5 trucks per day) program would be required, requiring increase digestion and CHP equipment. Therefore, an aggressive co-digestion program was not considered in this project. Based on the market assessment, the following recommendations were made: Identify a path for the WWTP to produce Class A biosolids Explore implementation of a moderate co-digestion program Explore implementation of CHP for improved energy recovery ALTERNATIVES DEVELOPMENT AND EVALUATION A number of alternatives were developed for initial screening. Initial screening reduced the number of biosolids and energy management alternatives to the following: Alternative 0 - Existing intended to capture the do nothing scenario, this alternative does not meet any of the City s goals, but was used to compare alternatives from a financial and logistical perspective. Alternative 1a Class B Cake (Base Case + CHP) includes critical reliability and redundancy improvements needed to continue mesophilic digestion and Class B cake production through the planning period. The reliability and redundancy upgrades would include the following: Copyright 2017 by the Water Environment Federation 6 962

7 New primary and secondary solids mechanical thickening using three belt filter presses in a new building One new mesophilic digester (same size as existing) Addition of one new centrifuge installed in the existing building (same size as existing) Addition of a cake transfer pump in the existing building (same size as existing) Addition of one new cake storage hopper (same size as existing) One new digester gas flare (same size as existing) A moderate co-digestion program One new CHP engine installed in a new enclosure This alternative was developed to demonstrate that achieving the City s core redundancy, capacity, and energy recovery goals for reliable operation will require investments in several unit processes. Alternative 1b Class B Cake (Base Case) includes reliability and redundancy improvements, similar to Alternative 1a, but without co-digestion or CHP. This alternative reflects a minimum investment required to maintain reliable operation. The municipality requested that this option be included in the evaluation, but it showed poor financial performance compared to Alternative 1a, so it was eventually removed from consideration. Alternative 2a Class A Cake via Thermophilic Digestion This alternative includes the same reliability and redundancy improvements as discussed for Alternative 1a. It also assumes that the two existing digesters will be converted to operate at thermophilic temperatures (135 degrees Fahrenheit [ F]). The conversion will require replacement of existing heat exchangers and other equipment upgrades. Additional digesters would be needed and would operate in a batch mode to meet Class A by time and temperature. This alternative meets the City s reliability and redundancy goals, as well as energy and resource recovery goals. This alternative can be phased in to maximize the use of existing infrastructure, is adaptable to future regulations, and offers diversity of end uses. The operational considerations for this option that have to be kept in mind are operation of the digester process at higher temperatures, which could be mitigated by implementing standard operating procedures (SOPs) and staff training; potential for increased nutrient loadings from sidestream, which could be mitigated by sidestream treatment; and in some instances, reported reactivation of pathogens and odorous cake, which could be addressed by odor control. Alternative 2b Class A Cake via Thermal Hydrolysis Process (THP) The advantage of this option is reliable, high-quality Class A Cake product with broad end-use opportunities in local markets. Furthermore, THP would increase the allowable solids loading to the existing mesophilic digesters. Therefore, no new mesophilic digester capacity would need to be constructed. However, THP would add significant complexity to the biosolids treatment process Copyright 2017 by the Water Environment Federation 7 963

8 because it would require addition of pre-dewatering, an additional cake hopper/transfer system, and steam boiler. It would also require an increased level of staffing, and likely a significant learning curve for plant staff. WWTP staff had safety concerns related to steam system operation, and the need for a full-time certified steam operator. The large heat demand for THP would reduce the amount of digester gas available for CHP. Therefore, microturbine units were assumed for this alternative because they are available at lower capacities, have higher heat efficiencies, and would be better suited compared to engines. Alternative CHP technologies could also be evaluated. This alternative meets the City s reliability and redundancy goals, as well as energy and resource recovery goals. Alternative 3b - Class A Dried Product This alternative assumes one drum dryer will be installed to process mesophilically digested and dewatered solids. If the dryer needed to be out of service, the plant would still be able to produce Class B cake through the existing digestion process. For this alternative, a new digester would be needed to provide reliability for the digestion process. This alternative could be phased-in and would integrate well in the existing mesophilic treatment processes, taking advantage of all of the sludge treatment processes that are currently in place. A key advantage of this option is a reliable, high-quality Class A pelletized product with broad end-use opportunities in local markets. This alternative also presents an opportunity for the WWTP to undertake marketing and distribution of its own biosolids product. Addition of a dryer would add some complexity to the biosolids treatment process, and would require an increased level of staffing. The risks associated with operation of a drum dryer, specifically operation at high temperatures and the potential for thermal events, were a concern to WWTP staff during workshop discussion. It was noted that the risk of thermal events can be managed and mitigated through design of safety systems, training, and operating protocol. The WWTP was interested in other ways to achieve a uniform product without the high temperature associated with a drum dryer. It was noted that a uniform granular product can be produced with a belt dryer that operates at lower temperatures followed by a pelletizer. However, the final product would likely be less desirable compared to uniform pellets. From an energy recovery standpoint, a dryer would use the large portion of digester gas, so less gas would be available for CHP. In addition, a dryer would also use natural gas, contributing to an increased carbon footprint for this alternative. Alternative 4 Third-Party Processing - captures opportunities for onsite third-party processing of the Class B cake into a Class A product, biochar, or other marketable material in exchange for a tipping fee. Based on a survey of local solids processing outfits, the tipping fee would likely be higher than the current fee that the WWTP is paying for solids hauling and disposal. This alternative includes the upgrades to the solids handling system included in the Base Case, but a third-party contractor would process all or a portion of the dewatered cake and market/distribute the final product. It was assumed that the third-party processing facility would be co-located at the WWTP. Copyright 2017 by the Water Environment Federation 8 964

9 This alternative offers numerous benefits, including adaptability to future regulatory changes and diversification of biosolids end use. It could be easily phased-in and integrated into the existing treatment processes, and would have minimal impact on existing operations staffing. However, depending on the technology selected, third-party processing may have large land requirements and is also likely to increase truck traffic to the WWTP if the third-party facility processes feed from other facilities. Risks associated with using a new technology also need to be considered, although technologies and their track record are improving. In addition, thirdparty contracting may have limited operational flexibility. Excess digester gas produced by the facility would be used by the third party and there would be limited opportunity for CHP. Therefore, CHP was not included in this alternative. The evaluation of this alternative indicated that there is currently no economic benefit to the engage other third-party biosolids management entities when compared to the current practice. Third-party processing opportunities should continue to be tracked and evaluated for economic and non-economic benefits because these opportunities are likely to become more accepted and economically viable in the future, and could provide an important path to diversification of biosolids end uses at the WWTP. Alternative 5 Combination of Class A Digestion and Thermal Drying - This alternative combines a Class A digestion process with drying capacity for one-half of the solids produced at the facility. Thermophilic digestion was assumed as the Class A digestion process, although THP could also be used. One dryer unit was used for evaluation purposes, with the assumption that if the dryer is out of service, the plant can still produce Class A biosolids. This is the most flexible and diverse option that combines the benefits and concerns that have already been discussed for the Class A cake and drying options. This alternative would provide the WWTP with the flexibility to be able to respond to future regulations and also to diversify the end product. It would also allow the WWTP to develop its biosolids marketing program as well as a variety of products to support it. However, it would also be most complex from the standpoint of implementation, staffing, operation and cost. Non-Monetary Evaluation Alternatives evaluation based on non-monetary criteria using the triple bottom-line model is shown in Figure 3. Alternatives that produce Class A product achieved the highest benefit scores as these provide the desired diversity of biosolids end use. Continuing with mesophilic digestion to produce Class B cake with added reliability and redundancy improvements (Alternative 1) had a lower benefit score. While this alternative would achieve reliable operation, the main concern is its inability to adapt to anticipated regulatory changes and alignment with the City s biosolids recovery goals. Copyright 2017 by the Water Environment Federation 9 965

10 Alternative 0 do nothing, achieved the lowest score. This alternative represents current conditions with significant reliability and redundancy deficiencies that would result in risky and unsustainable operation through the planning period. Of the alternatives that meet the City s reliability and redundancy goals, as well as the biosolids and energy recovery goals (2 through 5), Alternative 2a conversion to thermophilic digestion to produce Class A cake had the lowest life-cycle costs (Figures 4 and 5). These costs were comparable to alternatives that only addressed redundancy and reliability issues (1a and 1b), which made this alternative particularly attractive. In comparison, other alternatives that required addition of new treatment processes, in addition to improvements for reliability and redundancy (Alternative 2b, 3b, and 5), had higher life-cycle costs. Third-party processing, while requiring low capital investment, had the highest O&M cost due to higher tipping fees. Copyright 2017 by the Water Environment Federation

11 Figure 3. Comparison of alternatives based on non-monetary evaluation Copyright 2017 by the Water Environment Federation

12 Figure 4. Life cycle cost for evaluated alternatives Figure 5. Cost-to-benefit ratio for evaluated alternatives Copyright 2017 by the Water Environment Federation

13 Based on the above evaluation, Alternative 2a was selected as the preferred alternative. This alternative will provide a Class A biosolids product, which facilitates resource recovery and minimizes future biosolids distribution challenges. Because this alternative also includes a codigestion and CHP, it will also enhance energy recovery by offsetting power purchased from the utility. Investments in process equipment for conversion to thermophilic digestion were also lower than the other alternatives, resulting in a lower overall life-cycle cost. Considering the economic results and non-economic criteria scores, this alternative is expected to provide the biosolids and energy management approach that most effectively achieves the City s goals. IMPLEMENTATION PLAN Proposed upgrades to the WWTP biosolids processing and energy recovery systems required to implement conversion to thermophilic digestion are shown in Figure 6. Upgrades that would be implemented within the BEP planning period are indicated in orange, while future or optional processes that could be considered based on triggers are shown in blue. Copyright 2017 by the Water Environment Federation

14 Figure 6. Process flow diagram for the preferred biosolids management alternative conversion to thermophilic digestion Copyright 2017 by the Water Environment Federation

15 Four phases were identified to facilitate the transition and to integrate the BEP with the existing CIP: Immediate Needs include a new centrifuge, upgraded cake conveyance system, and new cake hopper. These projects address urgent reliability and redundancy needs. It should be noted that other workaround solutions to address redundancy may be considered. Phase 1 includes a new odor control facility and thickening system. These projects address reliability and redundancy concerns with the thickening system by providing a fully redundant and upgraded thickening process. Phase 1 also facilitates conversion to thermophilic digestion by reducing the hydraulic loading to the digester. Phase 2 includes a FOG receiving station, a new gas treatment and CHP equipment, and a new waste gas burner. These projects enhance energy recovery by increasing digester gas production and providing additional options for digester gas beneficial use. This phase also addresses redundancy concerns with the plant hydronic heating and waste gas burner system by providing standby units for both processes. Phase 3 involves modifications to the existing digesters to be able to operate at higher temperatures and construction of two new small digesters to convert the digesters from mesophilic to thermophilic operation. This phase increases resource recovery options by producing a Class A biosolids product, which have more distribution options than Class B biosolids. Phase 3 also increases the loading tolerance of the digesters to allow for higher hydraulic and solids loading rates compared to mesophilic digestion. Copyright 2017 by the Water Environment Federation 971