Wastewater to Energy: The Integration of Biosolids Drying Technology and Energy Recovery

Designed in the 1960's and constructed in the mid-1970's, the Milton Regional Sewer Authority's (MRSA's) 3.420 MGD wastewater treatment plant (WWTP) is in need of a complete facility upgrade. The upgrade will need to not only account for strict nitrogen and phosphorus effluent requirements, but also seek out opportunities to minimize energy dependence and operational costs through biogas production, electrical generation, biosolids drying and sale of electricity to the Grid.The end goal for this project is to create an energy independent, green wastewater treatment facility. The objectives are to maximize energy recovery by exploiting all reasonable sources; to minimize energy use throughout the facility; and to identify the most feasible methods of energy recovery. Anaerobic treatment will be used were applicable to meet these objectives through lower connected horsepower and biogas production.Anaerobic treatment helps achieve the goals of the project by reducing biomass production by 80% and produces about 7.6 ft3 of Biogas for every pound of chemical oxygen demand (COD) applied to the reactor. Approximately 624,600 ft3 of biogas (@75% methane) per day will be produced during the maximum month, enough to fuel two 633 kW generators. Each generator has a full load recoverable thermal output of 2.853 mmBTU per hour divided between three lower temperature waste heat streams (intercooler, lube oil, and jacket water) and one higher temperature waste heat stream (exhaust gas). Of this only the higher temperature exhaust stream is capable of supply the temperatures needed to evaporate water and dry sludge. The exhaust waste heat can provide approximately 2.62 mmBTU to the drying process.There are two basic types of biosolid dryers on the market. The first is a direct dryer that utilizes direct contact between hot gas (heat-transfer medium) and the biosolids. The second is an indirect drier where the heat-transfer medium is separated from the biosolids by a metal wall. Several configurations exist for each basic type of dryer; this project looks at a belt-type direct dryer and a paddle-type indirect dryer.When determining the ability and/or feasibility of biosolids drying for this project two heat exchanges are important. First is the waste heat to heat-transfer medium via a tube and shell heat exchanger and the second is the transfer of heat from the heat-transfer medium to the biosolids in the dryer. Each step is important and will help to determine the overall process efficiency and ability to utilize waste heat. Both heat exchanges follow the basic equation:Q = U A (LMTD)Where,Q is the quantity of heat to be transferred per unit time, BTU/hrU is the heat transfer coefficient, BTU/(hr-ft2-F)A is the available heat transfer area, ft2LMTD is the log mean temperature difference, FThe heat transfer coefficient is the most complex factor in this equation as it is a sum of all individual thermal resistances and varies with temperature, material of construction, material thickness, area of heat exchange, and others. For each type of dryer the heat required to dry biosolids is between 1400 BTU/ft3 and 1700 BTU/ft3, this include the sensible heat associated with raising the wet biosolids to 212 F, the heat of vaporization, and heat associated with raising the dry biosolids above 212 F. The largest component is the heat of vaporization, which accounts for roughly 85% of the heat required.The more efficient system for using waste heat is generally the direct dryer, requiring 1450 BTU/ft3 versus 1650 BTU/ft3 for indirect dryers. The major difference is the method of heat transfer. Efficiency is lost as heat transfer occurs between the heat transfer fluid, internal film, metal wall, product film, and product. This leads to a 14% increase in BTUs required. For many small and medium sized facilities the reduced efficiency is not enough to warrant the increased capital cost associated with direct drying.For the MRSA project, the generators will produce enough usable waste heat to dry approximately 6.0 dry tons of sludge per day, an evaporation capacity of 21.7 tons of water. An indirect dryer has capacity to dry approximately 5.25 dry tons per day, an evaporative capacity of 17.7 tons of water. The annual expenses, including operation, maintenance and debt service, are 271,500 for the direct dryer and 187,300 for the indirect dryer. The difference is 84,200 annually; however the direct dryer has capacity to accept 5.5 wet tons per day of hauled-in dewatered biosolids. At estimated revenue of 35.00 per wet ton hauled-in biosolids will generate 70,300 annually.What this means for Milton is a selection between capacity and cost, between direct and indirect dryers. This project demonstrates how it is possible to convert an energy intensive wastewater treatment process into a “Green” wastewater treatment process by efficient anaerobic treatment and wastewater to energy processes.