Combined heat and power and a host of other energy innovations mean $750,000 in documented savings — so far.
When the Rochester (Minn.) Water Reclamation Plant needed to expand, plant manager Chet Welle wanted to find long-term savings to help offset the cost to customers. “Energy savings was one of the things I really focused on,” he says.
In the five years since the $4.7 million expansion, which boosted capacity from 19.1 mgd to 23.85 mgd, the plant has received $253,000 in rebates from the local natural gas utility, Minnesota Energy Resources. Each rebate was equal to the demonstrated savings from one year of operation of biogas and heat recovery equipment added after the expansion.
However, it is the ongoing energy savings that will mean the most to customers — $750,000 a year documented so far, or about 10 percent of the plant’s $8 million annual operation and maintenance budget.
The plant consists of two parallel systems sharing a headworks. Influent is split between the original two-stage high-purity oxygen (HPO) activated sludge treatment process built in 1982, and the new single-stage activated sludge treatment process with conventional aeration. The effluent from both is combined for chlorination and dechlorination before discharge to the Zumbro River. Its two 1.85-million-gallon anaerobic digesters are fed from a common solids handling facility.
Combined heat and power has been used since the plant was built. The original system consisted of two 400 kW biogas engine-generators with exhaust gas and jacket water heat recovery. The first of the old engines was replaced in 2002, and the other in 2009, with new models 20 percent more efficient.
The Waukesha 1,000 kW lean-burn turbocharged engine-generators (GE Energy) are dual-fuel models that burn both biogas and natural gas. The projects included improvements to the jacket water and exhaust heat recovery systems, along with controls and switchgear, costing $3.8 million in all.
A new exhaust gas heat recovery boiler was also installed in 2009 after an old boiler failed soon after installation. The new boiler cost $291,000, but also brought a rebate of $129,650 after its savings were documented. It was so effective that the plant added another such boiler to the second engine in 2010 and earned a $99,650 rebate.
The financial payback for the boilers was 1.5 years, while the engine work has a payback of 9.4 years. “Emissions are also much better,” adds Welle. “The emissions from two 1,000 kW engines are less than they used to be from one 400 kW engine.”
With biogas production of 338,000 cubic feet per day (66 percent methane), the two engine-generators can continuously generate from 550 to 700 kW. The local electric utility, Rochester Public Utilities, provides various rate credits for a total savings of about $260,000 a year. The plant also uses the generators for in-house peak shaving.
Natural gas savings
Biogas generation and heat recovery saves the plant nearly $370,000 a year over the cost of natural gas, 2.5 million Btu/hour from engine jacket water, and 2 million Btu/hour from exhaust gas heat.
“At 180 to 190 degrees F, it goes into a hot water transfer loop that covers the whole complex of 27 buildings on the eight acre site, including digester heating,” says Welle. “Everything is heated by waste heat recovery for much of the year. We supplement with natural gas in the coldest part of the year.”
And work continues. In late 2012, the natural gas utility provided a rebate of $23,886 after the plant demonstrated a year of energy savings from insulating the roofs of its two digesters. Future plans include a FOG and high-strength industrial waste program and more biogas enhancements.
The savings don’t stop there. The facility has used effluent thermal recovery since 1982, though the savings couldn’t be documented at the time for a lack of instrumentation. A second set of final effluent heat exchangers was added during the 2007 expansion, along with instrumentation that showed heat recovery equaled a natural gas savings of $78,000.
“The effluent temperature is about 58 degrees F in winter,” says Welle. “It might get down to the low 50s in a really cold stretch of weather. In summer, it runs around 62 degrees and could get as high as 70.”
The effluent is used to cool all gas and air compressors and the high-purity oxygen air compressors. It also runs through HVAC coils to reduce heating and cooling costs, provide a heat sink for summer cooling, and preheat buildings in winter.
The plant has just added a new train of effluent thermal energy for heating the primary clarifier. Effluent that enters at 55 degrees goes through the HVAC preheat coils, a heat pump, and the HVAC secondary coils, and brings 77-degree air to the clarifier. Welle believes the savings could be as high as $150,000 a year.
“The savings are real, but nothing is free,” says Welle. “It doesn’t work without a lot of cooperation. If you don’t have a good SCADA system and make seasonal adjustments, it’s not going to work. The staff running it right is what really saves you the energy.”