Paradigm Shift to “Water Resource Recovery Facilities”
Impacts to / Opportunities at WSSC
The water industry “Utilities of the Future” are moving toward “Resource Recovery.” Multiple reports outline strategies and technologies that wastewater utilities can implement to reach this goal. Historically, the wastewater industry has focused on protecting water quality through relatively high energy treatment technologies and chemical use (e.g., substantial aeration energy required for enhanced nitrogen removal; internal recycle pumps for additionally enhanced nitrogen removal; methanol dosing to enhance denitrification; aluminum sulfate use for chemical precipitation). With the construction of the Bio-Energy facility, the Washington Suburban Sanitary Commission (WSSC) will be implementing major technologies that will substantially move our plants further forward as “Utilities of the Future.”
Currently, WSSC’s five major water resource recovery facilities (WRRF):
- recover water;
- return the clean water (that has gone through advanced treatment and enhanced nutrient removal) to the environment;
- reuse a small portion of the water on-site for plant purposes;
- beneficially reuse biosolids as a Class B fertilizer (except Western Branch); and
- renewable energy (wind and solar) is used to provide a significant portion of power to the facilities. A design-build project is currently underway to construct a new Bio-Energy facility at Piscataway, which will recover more resources from the wastewater in the form of heat and renewable natural gas suitable to help power the Piscataway plant. This facility should be coming on-line in 2021.
Figure ES-1 below depicts where WSSC is currently (Stage 1) and summarizes a roadmap for additional resource recovery opportunities in the future.
Figure ES-1. Proposed Resource Recovery Map
- Continue forward with the Bio-Energy project at Piscataway - A design-build project is underway to construct a centralized facility to anaerobically digest approximately 60 dry tons/day of biosolids generated by the 5 “major” WRRFs. The methane generated by the digesters will produce approximately 1.6MW of “green power” while reducing the biosolids quantity by 40-60%; this will be supplemented by natural gas to produce up to 2-3MW total power for Piscataway. The waste heat from the natural gas generators will be used to run the thermal hydrolysis system, located upstream of the digester, which will help break down the biosolids and increase their digestibility. Overall combined heat and power energy efficiency are expected to be 60%-70%. As a result of the thermal hydrolysis process, WSSC biosolids will be upgraded from Class-B to Class-A, and there will be fewer restrictions regarding its distribution and ultimate use. As a result of the digestion process, a fair amount of nitrogen and phosphorous will be released from the biosolids, and will increase the loads back to the Piscataway WRRF; however, sidestream deammonification processes utilizing new annamox bacteria technology will be used to pretreat the return flows using considerably less air, chemicals, and energy than the facility’s mainstream process would use.
- Optimize energy use – Continue to look for ways to optimize treatment processes and identify energy improvements. Energy audits have been conducted for each of the wastewater plants, and areas of potential improvement have been identified. Multiple energy improvement projects have been initiated.
- Consider alternative carbon sources to enhance denitrification (e.g., MicroC 3000 instead of methanol) – Parkway is already using MicroC 3000, and if the rest of the wastewater plants switched to MicroC 3000 (or another cost-effective product), the Commission could save $400+k/year at current methanol/MicroC 3000 rates. a reclaimed waste as its carbon source to help drive the denitrification process (and Western Branch and Seneca have since switched as well). Use of reclaimed products is a better use of resources and reduce greenhouse gas impacts. This was a relatively seamless switch because no capital improvements would be needed.
- Evaluate primary treatment/carbon diversion for applicable plants (primary treatment at Seneca and possibly Western Branch; (i.e., carbon diversion at Seneca and possibly Western Branch; as well as consideration of chemically enhanced primary treatment at Parkway and Piscataway). – Reduces aeration energy requirements, enhances biogas production and reduces residual solids to be land applied. Carbon diversion prior to the “secondary treatment” processes reduces aeration energy demands, enhances biogas production, and reduces residual solids to be land applied. Tradeoffs include increased supplemental carbon and alum doses. Would likely require significant modifications at Seneca. Preliminary operating cost analysis conducted in-house suggested net operating cost savings (capital costs not yet determined). Recommend using a BOA consultant to conduct a more detailed evaluation. Cost-benefit evaluations and bench-scale trials are currently underway for Seneca.
- Add enhanced biological phosphorus removal (EBPR) where possible – Reduces alum chemical cost and hauling cost (total alum cost is about $800,000/year; could increase to about $1 million/year once Parkway no longer receives Patuxent solids). Aligns WSSC to potentially recover phosphorus from biosolids in the future. Reducing alum use at the plants would also likely increase biogas production. An EBPR feasibility study is underway (as of June 2017), and the consultant believes EBPR is feasible and cost-effective at all of our plants. Pilot testing and more detailed evaluations would be needed in the future because this would likely require significant modifications at the plants. The Engineering and Environmental Services Division (formerly Technical Services Group) may have the capacity to fund pilot testing at Damascus. Additional evaluations could use an existing BOA contract, or ESP funding may be requested. EBPR could reduce alum chemical costs and biosolids hauling costs. Reducing alum use at the plants would also likely increase biogas production. An EBPR feasibility study was completed and determined that EBPR is feasible and cost-effective at all of our plants. The inline fermentation option is already being tested at Seneca and Western Branch. Additional pilot testing at Parkway and Piscataway has been proposed, and more detailed evaluations will be needed in the future to determine the most cost-effective strategies and to evaluate/mitigate impacts downstream at the future Bio-Energy facility.
- If EBPR is implemented, evaluate phosphorus recovery – Phosphorus is a critical and limited resource. Can recover marketable struvite product to generate revenue. Also reduces phosphorus concentration in the biosolids, potentially reducing land application limitations. On-hold until EBPR implementation underway. Eventually, ask a BOA consultant to compare available phosphorus recovery technologies. Phosphorus is a critical and limited resource. If EBPR is successfully implemented, WSSC can recover marketable struvite product to generate revenue, but more importantly, conserve an important resource. Recovery will also reduce phosphorus concentrations in the biosolids, potentially reducing land application limitations.
- Evaluate co-digestion of food waste and other high strength wastes – Co-digestion can improve the volatile solids reduction, biogas production, and energy production substantially. Impacts to the nitrogen and phosphorus performance of the Piscataway would also have to be evaluated. Addition of fats, oils, & greases (FOG), food wastes, and other high strength wastes to the Bio-Energy digesters, to co-digest with the biosolids at the Bio-Energy facility, can substantially improve the volatile solids reduction, biogas production, and energy production. Impacts to the nitrogen and phosphorus performance of the Piscataway plant would have to be evaluated.
- Consider mainstream shortcut nitrogen removal and/or efficient oxygen transfer processes – Reduces aeration energy requirements. Shortcut nitrogen removal can also reduce supplemental carbon chemical costs and solids hauling costs, and could help offset chemical costs associated with carbon diversion (recommendation #4). Requires very good aeration controls (blowers, ammonia/nitrate+nitrite probes, logic). Research by others underway. WSSC to monitor and look for opportunities for our plants, especially after the plants undergo aeration control system upgrades. Possible that capital improvements would not be needed or would be minimal with an existing advanced aeration control system. Shortcut nitrogen removal, involving annamox bacteria, substantially reduce aeration, supplemental carbon, and solids hauling costs. Research by others underway. WSSC to continue monitoring and looking for opportunities for our plants.
- Continue to expand use of renewable energy sources (e.g., wind, solar, co-digestion, microhydro, thermal energy recovery). Previously WSSC purchased 85% of a 29.1MW wind farm in Pennsylvania over a 10-year period; that contract expired at the end of 2017. WSSC is now preparing an RFP for a new wind contract and is committing to purchase 30% of its annual electrical energy consumption from the new wind source. In the interim, WSSC is purchasing wind “Renewable Energy Credits (RECs) for 100% of its electrical energy requirements. In addition, WSSC has purchased power from 2MW solar sites at both the Seneca and Western Branch WRRFs, and WSSC is currently negotiating Purchase Power Agreements to construct another 2MW solar facility at Seneca, and another 4MW solar facility in Prince George’s County.
WSSC’s Patuxent Water Filtration Plant has a long history of relying upon hydraulic turbines to pump water from the reservoir to the plant when sufficient water is present in the reservoir. In addition to these, a couple new minor micro hydro opportunities have been identified for further evaluation. Evaluations have shown that although the WRRFs have a steady flow of effluent, there is insufficient elevation drop to warrant investment in hydropower.
Although it is reported that influent wastewater contains more energy than what is required to treat the wastewater, and there are goals for “Net Zero Energy Use”, over 80% of the influent wastewater energy is in the form of low-grade waste heat. The potential to recover some thermal energy does exist, but implementation is severely inhibited by the lack of existing “district heating loops” and nearby high-density population centers which can/will use the recovered heat.
- Look to expand water reuse opportunities – Up to 5 mgd of Piscataway’s final effluent may be used as cooling water by a power plant in the next few years. WSSC can look for other industries or groups in the area that may be interested in non-potable water reuse. This region is currently not experiencing a water shortage, which may decrease the demand for water reuse at this time. Currently Damascus and Seneca WRRF final treated effluents are discharged into tributaries of the upper Potomac River, and further downstream are the intakes of WSSC’s Potomac Water Filtration Plant, as well as the intakes for various northern Virginia and District of Columbia water filtration plants. Very broadly defined, the Damascus and Seneca WRRFs could be described as indirect potable reuse facilities. Most recently a project has been undertaken to divert 5-10 mgd of Piscataway’s final effluent for use as cooling water at a power plant. WSSC continues to look for other industries or groups in the area that may be interested in non-potable water reuse.
- Evaluate additional Biosolids-to-Energy technologies after Bio-Energy facility is in operation – Although WSSC biosolids will be upgraded to Class A biosolids and reduced in half after anaerobic digestion, WSSC will still need to land apply about 14,000 dry tons per year in 2020. It is possible that the remaining biosolids could be upgraded to an additional beneficial product (e.g., upgrade the Class A biosolids fertilizer to biocrude). Additional pilot testing and evaluation is recommended after 2021.