The Role of Anaerobic Digestion in Wastewater Management

Wastewater systems contribute to significant negative impacts not only on a regional water body, but also to global energy, climate, and sustainability. In thinking holistically of water and wastewater management, energy recovery from wastewater becomes an appealing option to achieve greater resource recovery1, 2. The most common form of energy recovery is Anaerobic Digestion, which is the biological degradation of organic matter in the absence of oxygen with subsequent conversion of chemical energy in organic carbon into biogas3. Typically, Anaerobic Digestion has been used with wastewater sludge treatment and reduction, agricultural manure management and food waste management 3. To accomplish more sustainable resource recovery and reduce the overall energy footprint, wastewater can be regarded as a renewable resource for converting embedded chemical energy into biogas 4. In the United States, currently there are 16,000 publicly owned wastewater systems. Only 544 of these use Anaerobic Digestion 3. That means there are at least 15,000 facilities that send their sludge to landfills or incinerators which contributes to global warming and air pollution. The importance of Anaerobic Digestion arises with the ability to convert the organic compounds in waste into biogas. Biogas is comprised of 60%–70% methane, 30%–40% CO2 and a small percentage of trace gases. A combined heat and power (CHP) engine can use this biogas to create electricity and heat, or the compressed biogas can be used as fuel to power fleet vehicles. Compressed methane or natural gas is often viewed as a cleaner alternative to diesel fuels 5. With this ability to create electricity and fuel, wastewater treatment plants (WWTPs) have the potential to not only power their entire facility, but also receive revenue by sending excess electricity to the grid. Currently, a few WWTPs are sending electricity back to the grid. Whereas, most WWTPs that use Anaerobic Digestion as one of the treatment process, either use produced biogas to heat their buildings, heating influents and flare the excess biogas or profit from “tipping fees” paid by other local companies. A tipping fee is a fee charged for the amount of waste disposed to a landfill 7, 8. Not only can Anaerobic Digestion create energy from waste, but through the Anaerobic Digestion process, digestate that is the material remaining after the Methanogenesis stage is called “biosolids”. The biosolids can be further treated to produce higher quality biosolids either grade A or B which can be sent to local farms or nursery stores as a fertilizing compound or as soil conditioner. The Co-digestion Economic Analysis (Co-EAT) model developed by US EPA allows the user to input current operating parameters which tailors the model to plant specific operations9. This model is designed to quantify the impacts associated with adding co-digestion onto an existing Anaerobic Digestion system. The Co-EAT model can predict the quantity of biogas production based on volume of volatile solids (VS) destroyed daily or annually9. Furthermore, the model can also estimate economic parameters such as tipping fees, market value of the biogas and associated disposal costs. It can also compare these economic and physical characteristics under a variety of differing scenarios. For this contribution, this model has been applied to the following two case studies.