Growing Up Together: Sidestream Startup Concurrent with THP Digestion to Meet Strict Recycle Limits

WSSC WATER is nearing completion of the Bio-Energy project to thermally hydrolyze and anaerobically digest all biosolids from their five Water Resource Recovery Facilities (WRRFs) at a centralized facility to produce a Class A biosolids. As part of this project, thermal hydrolysis pretreatment (THP) and mesophilic anaerobic digesters are being constructed at the Piscataway WRRF to provide the necessary stabilization to meet regulatory requirements as well as produce valuable digester gas, which will be converted to renewable natural gas (RNG) suitable for pipeline injection. See Figure 1 for an overall process schematic. The Bio-Energy project will fundamentally change how WSSC WATER manages biosolids, allowing a focus on long-term sustainable end products with a Class A dewatered cake biosolids, and RNG suitable for pipeline injection and use as vehicle fuel. The project will reduce the greenhouse gas emissions and result in significant operational cost savings. But with these benefits comes one of the biggest challenges of the Bio-Energy Project to treat the additional nutrient recycle loads that will be generated from sludge imported from the other WRFs and increased even more by THP and anaerobic digestion. Total nitrogen recycle loads from the Bio-Energy Project must be limited to ensure the Piscataway WRRF can maintain compliance with effluent discharge limits. A two reactor ANITATM Mox sidestream treatment process will startup concurrent with the digestion process to treat high ammonia filtrate from the Bio-Energy dewatering process. Managing filtrate generation in coordination with startup of both the Digester and Sidestream processes is essential to maintaining process compliance at Piscataway WRRF. Figures 2 and 3 present digester and sidestream treatment startup graphs. This presentation discusses the integrated startup plan for anaerobic digestion and sidestream deammonification at the Piscataway WRRF to meet multiple goals: - Stable startup of both biological systems, minimizing potential for digester souring, biological washout, etc. - Limiting nitrogen recycle to Piscataway WRRF - Expediting the commissioning schedule as much as practical to allow WSSC WATER to reap the benefits from this facility as soon as possible. Phase 1 Preparation and Seeding Phase 1 involves preparation and biological seeding of both digestion and sidestream deammonification, initially with one digester and one reactor. - Digester 1 will be partially filled with disinfected utility water to a minimum level to initiate pump mixing. The headspace will be purged with nitrogen to prevent an explosive atmosphere as methane concentrations increase. Steam will be used to bring the water to the mesophilic temperature range. Then liquid seed sludge will be imported from the DC Water Blue Plains Advanced Wastewater Treatment Plant (WWTP), which is thermally hydrolyzed and Class A. The digester is now ready to receive hydrolyzed sludge from the Bio-Energy THP process. - Sidestream deammonification has a similar preparation process. The first treatment reactor will be mostly filled with virgin media. Process flow from the WRRF will be introduced to the reactor to condition the media and promote biological attachment. Media conditioning will continue into Phase 2 - Digester Ramp Up to Piscataway Load. Once conditioning is complete, media with annamox biology from an operating ANITATM Mox process will be introduced to the reactor. The sidestream deammonification process is now ready to begin receiving filtrate. Phase 2 Digester Ramp Up to Piscataway Load Indigenous sludge from the Piscataway WRRF will be thickened, screened and dewatered prior to THP. THP throughput will be increased from approximately 4 dry tons/day (dtpd) up to the full Piscataway load, approximately 16 dtpd over approximately 30 days. During this ramp up, the ratio of volatile solids feed to volatile solids under digestion will be kept below 10%. At the end of Phase 2, Digester 1 will reach its normal operating level. Digester 2 will be prepared with water, nitrogen purge and heating before Digester 1 contents are split to Digester 2. Digestate will not be withdrawn during Phase 2 and no filtrate will be sent to the sidestream deammonification process. Phase 3 Sidestream Reactor 1 Startup and Initial Acclimation During Phase 3, digester feed will be held constant at the Piscataway WRRF load (16 dtpd). This period will be used to build the volume under digestion, establish robust mesophilic anerobic digestion microbial communities, and fine-tune operations prior to introduction of imported sludges. Digester withdrawal also begins during Phase 3 to generate filtrate to be used in the Sidestream Deammonification Reactors biological startup. Filtrate sent to the sidestream deammonification process will increase in both concentration and mass loading of ammonia (nitrogen), allowing the annamox bacteria to grow and acclimate to THP filtrate, increasing treatment capacity. During Phase 3, ammonia concentrations in the digesters and filtrate will also be increasing, and will be accounted for when calculating digester withdrawal and filtrate generation and dilution rates into the sidestream reactor. The sidestream process will start in moving bed biofilm reactor (MBBR) mode, with any sludge collected in the clarifier wasted to drain. Phase 4 Sidestream Reactor 1 Acclimation and Reactor 2 Preparation Digester 1 and 2 continue building the volume under digestion, establishing robust mesophilic digestion microbial communities, and fine-tuning operations during the 16 dtpd hold period. The influent ammonia concentration and load will be gradually increased to sidestream reactor 1. Dilution water will be adjusted to meet target feed concentrations and the mass loading will be gradually increased. RAS recycle will be initiated, and the sidestream process will convert from MBBR to integrated fixed-film activated sludge (IFAS) mode. Sidestream Reactor 2 will be prepared for startup, including conditioning of virgin media, introduction of seeded media and phased influent loading. Earlier startup of sidestream Reactor 2 represents an optimization from the initial startup plan, which deferred sidestream reactor 2 startup until reactor 1 was fully acclimated and performance tested. Phase 5 Increased Digester Loadings, Sidestream Reactor 1 Fast Acclimation and Reactor 2 Initial Acclimation After the digester 16 dtpd hold period, sludge will be imported from the four remote WSSC Water WRRFs to increase sludge to the Bio-Energy process. Both digesters will reach normal operating level, and digester withdrawal will be carefully managed to ensure filtrate does not overwhelm the sidestream reactor treatment capacity or exceed total allowable Bio-Energy recycle limits. While there is additional in-tank volume reserved for rapid volume expansion, once the digesters are full feed volume will generally match withdrawal volume, so sidestream treatment capacity must be sufficient to maintain total recycle load below plant limits. Sidestream Reactor 2 seeding and phased influent loading will commence at the conclusion of the Sidestream Reactor 1 slow acclimation period, and follows the same approach as reactor 1. Phase 6 Steady State Digestion and Sidestream Deammonification Phase 6 begins as the Bio-Energy process reaches full sludge load, with all indigenous and remote sludge being treated through THP and digestion. Both digesters will be in steady state operation. As Sidestream Reactors 1 and 2 complete ramp up to full treatment load capacity; startup will be complete, sidestream will be in steady state, and final operation and performance testing will commence. Conclusion Sidestream deammonification is critical to the successful implementation of WSSC Water's Bio-Energy Project, mitigating the impacts of increased nutrient recycle from THP and digestion with imported sludge from WSSC Water's remote WRRFs. The concurrent biological startups of sidestream treatment and digestion must provide stable startup of both biological systems while staying below nutrient recycle targets, all while maintaining an advantageous startup and commissioning schedule. The detailed planning undertaken for this facility allows for all parties to be fully informed and prepared moving into this integrated startup process.