Operators of a US wastewater treatment plant have seen and acted on the opportunity to import extra dairy industry waste in order to increase power generation capacity on site. The project has delivered benefits to all parties, as George Bevington, Robert E Ostapczuk and Paul C Bassette report.
The Gloversville-Johnstown Joint Wastewater Treatment Facility has served the cities of Gloversville and Johnstown in upstate New York since 1970.
In early 1992, the facility had just completed a major improvement project that increased the secondary treatment capacity and added anaerobic digestion and sludge dewatering processes to accommodate industrial flows and loads from the leather tanning and finishing industries. These improvements created a facility that treated wastewater that was 70% from industrial and commercial sources and 30% from residential sources.
Over the next several years, the leather and tanning industries slowly and steadily closed operations in the cities of Gloversville and Johnstown, leaving a facility with excess capacity and mounting debt service for the remaining users. Like many areas of upstate New York, traditional manufacturing industries opted to move operations overseas during this time period in an effort to reduce costs associated with labour, environmental regulations and taxes.
|One of two 350 kW engines commissioned in the summer of 2010 in a recent upgrade at the Gloversville-Johnstown plant|
Declining revenue with increasing costs due to more stringent regulations, inflation and other costs made for a difficult financial situation for the facility through the early 2000s. This situation resulted in large rate increases for two communities that were losing jobs as the leather and tanning industries moved overseas. As financial issues were mounting and heading to a crisis, facility management sought ways to reduce operating expenditure without affecting environmental performance of the facility.
In 2002, improvements to the aeration system resulted in the facility’s first step towards achieving a more sustainable wastewater treatment facility (WWTP). The improvements included ceramic and flexible membrane fine bubble diffusers, reducing the blower capacity and incorporating on demand dissolved oxygen control that delivered oxygen to the aeration basin zones with real time demands. This first project resulted in annual energy reduction of 1500 MWh per year. The project planning phase was partially funded by the New York State Energy Research & Development Authority (NYSERDA) through the Flexible Technical Assistance (FlexTech) programme.
Since that initial energy conservation project, the WWTP management has sought ways to reduce operating costs without compromising environmental performance. A hauled waste programme was expanded to increase revenue and utilize the unused capacity at the WWTP. In 2003, the WWTP began accepting 45,000 to 61,000 litres per day of dairy whey that was treated in the anaerobic digesters. This was the beginning of co-digestion at the WWTP, providing tipping fees for acceptance of the added wastewater and increasing biogas and electrical power generation.
After the initial dairy loads from a cheese producer were accepted via tanker truck, two dairy processing facilities located in the industrial park adjacent to the WWTP. The first, Euphrates, a feta cheese producer, hauled whey to the WWTP and process wastewater was discharged to the municipal sewer system and treated in the conventional wet processes at the WWTP.
With the success of treating dairy wastewater, the WWTP was able to attract FAGE, a Greek yogurt producer, to the industrial park. Johnstown, New York, was an ideal location for a yogurt producer, offering access to large northeastern US markets as well as proximity to the milk supply required to manufacture the yogurt. However, the capacity of the WWTP and historical experience treating volumes of high-strength industrial wastewater was the ultimate factor in FAGE’s decision to locate in upstate New York.
The marketing of the facility was successful, and the result was the reduction of energy consumption and increasing of on-site electrical generation with existing 20-year-old equipment. This meant that 35% of the electrical power requirements for the WWTP was generated by the existing engines. But production increases projected by FAGE indicated that an upgrade was needed to accommodate its growth in a more cost-effective manner by the WWTP.
The two old engines were manufactured by CAT; they each drove a 150 kW induction generator. With biogas, the actual output of these engines was rather less, around 110 kW. Both engines were operated on (and rebuilt several times) over the 20-year period. The engines had heat recovery (engine jacket and exhaust heat) that was used to provide heat to the digester. The engines, generators, controls, and heat recovery piping were all demolished during the engine upgrade.
2008–2010 UPGRADE PROJECT
The facility commissioned engineering firm Malcolm Pirnie (the Water Division of ARCADIS) in 2008 to prepare a feasibility study to develop solutions to treat the additional flows and loads through the existing two-stage mesophilic anaerobic digesters and the existing conventional activated sludge process. A comprehensive design was completed and, after competitive public bidding, contractors began construction of the systems required to handle the projected increased high-strength wastewater loadings from the dairy sector. The main design principle was sustainability in a systematic attempt to harness the power generating potential of the high strength wastewater.
An industrial sewer, dedicated to the dairy processing industries in the adjacent industrial park, conveys approximately 150,000 gallons per day of process wastewater to a dairy wastewater pretreatment facility located at the WWTP. The pretreatment facilities include a microstrainer and dissolved air flotation thickeners to reduce biochemical oxygen demand (BOD).
The underflow is pumped to the primary clarifiers and the float is pumped to the sludge holding tank. The high-strength float is ultimately pumped to the primary digester for conversion to biogas. The system became operational in early 2011.
Dairy whey is pumped directly from one of the dairies through a dedicated forcemain from the adjacent industrial park to the WWTP. There are two glass-lined whey equalization tanks at the WWTP. Whey from the industrial park can be directed to either tank. All whey is pumped directly to the anaerobic primary digester from these storage tanks in a continuous, 24/7 manner.
|Figure 1. Performance of the CHP plant during 2010|
The sludge thickening system is designed to process all sludge to a minimum solids concentration of 6%. The thickened sludge is pumped to the primary digester. Gravity belt thickeners also send thickened sludge to the primary digester, reducing water content, thus reducing the heating requirements to maintain digester temperature at 37ºC.
Mesophilic anaerobic digesters
The primary digester is 27 metres in diameter and operates with a side water depth of approximately 10 metres. The digester has an effective volume of approximately 5700 m³. Sludge draw-off piping is located at various elevations within the digester. An unvalved overflow is provided for the gravity transfer of sludge out to the secondary digester.
The secondary digester is 27 metres in diameter and operates with a side water depth of approximately 8.5 metres. The digester has an effective volume of approximately 4900 m³. Sludge draw-off piping is also located at various elevations through the digester. Sludge is drawn off the bottom of the digester and flows by gravity to the dewatering day tank.
Both digesters are mixed with confined gas mixing systems. The systems each comprise seven gas mixers located throughout the digesters and a liquid ring compressor. Biogas is stored in a dual membrane gasholder that is located directly behind the digester complex. The gasholder has an approximate working volume of 1300 m³.
Combined heat and power systems
Biogas fuels two 350 kW synchronous engine gen-erators, manufactured by Caterpillar. The engine generators can operate either in parallel with the utility or in an island mode (that is, independent of any other power source). The island mode allows the generators to provide power to the WWTP during emergency conditions, should utility power be lost. Waste heat is recovered from both the engine jacket and the exhaust. At 100% load the CHP system can recover approximately 410 kWh (1.4 MMbtu per hour), which is sufficient for the primary digester and building heating loads. Virtually all digester heating is accomplished from waste heat from the 350 kW engines.
Prior to digestion, the primary sludge is conditioned with ferrous chloride for odour reduction and to bind sulphur in the sludge. The biogas is compressed to 5 psi and water and particulate matter are removed through a coalescing filter and desiccant filter prior to utilization in the engine generators. The upgraded CHP system became fully operational in October 2010.
PERFORMANCE OF DIGESTOR/CHP PLANT
The combined result of all of the upgrade project systems is that the electrical generation capacity at the WWTP was increased from 270 kW to 700 kW.
When the dairy whey is anaerobically digested, carbon dioxide, methane and trace gases are generated. Typical carbon dioxide concentrations in the biogas vary from 40% to 45%, and the methane content averages 55%. At that level, combustion by the Caterpillar engines is easily maintained.
Currently, the WWTP is not permitted by the electric utility to net meter back to the grid; however, all electricity generated on site avoids an electricity charge of approximately $0.12 per kWh.
Engine commissioning occurred during the summer of 2010, and by mid-October the engines started running 24/7. On average, from 90% to 95% of the electricity required to operate the facility is generated by the 700 kW engines. Figure 1 illustrates the performance of the CHP plant during 2010.
COST AND GRANTS
The improvements to the whey equalization, dairy process water pretreatment, sludge thickening, sludge dewatering facilities, anaerobic digestion, and engine generators were required to process the dairy waste. The total cost of the project was $10.6 million.
Because of the job creation at FAGE, the WWTP was award a $2.2 million US Economic Development Agency (US EDA) grant in 2008. NYSERDA provided a $1.0 million grant from the Anaerobic Digester Gas to Electricity programme and a $400,000 grant for full-scale operational demonstration of the recuperative thickening loop. As a result of its generation of renewable energy from biogas, the project also received an additional $6.0 million Green Innovative Grant Program (GIGP) from the New York State Environmental Facilities Corporation (NYS EFC) as funded through the American Reinvestment and Recovery Act (ARRA).
Grant funding totalled $9.6 million, leaving $1.0 million to be financed by the WWTP, which results in a simple payback of just over one year
In addition to financial sustainability, the project is resulting in a greenhouse gas emissions reduction of 1950 tonnes of carbon dioxide per year. With the financial support of NYSERDA, the WWTP has relinquished its rights to Renewable Energy Credits (RECs) for the next three years. These RECs are projected to have a value of approximately $100,000 per year (assuming $0.02 per kWh).
Co-digestion of dairy wastes at the Gloversville-Johnston Joint Wastewater Treatment Plant is resulting in a sustainable and symbiotic relationship between industry and the municipal WWTP. The dairy companies have an economical disposal method of high-strength wastes and are reducing their environmental risk by not having to treat their own wastes at the source. They can focus their efforts on making their products.
The WWTP benefits from a new revenue source and a significant supply of biogas to produce heat and power. Instead of only regulating an industry at the source through an industrial pretreatment programme, the WWTP now pretreats dairy wastewater on site and continues to focus on its core mission, the treatment of wastewater. The conversion of high-strength wastewater into energy was accomplished through the system upgrade.
George Bevington is the Manager of the Gloversville-Johnstown Joint Wastewater Treatment Facility, Johnstown, New York, US. Email: email@example.com
Robert E Ostapczuk and Paul C Bassette are both with Malcolm Pirnie Inc.
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