Proposal for the treatment of Biosolids at the Ann Arbor WWTP


The Ann Arbor Waste Water Treatment Plant handles 19,000,000 gal/per day of municipal sewage separating raw sludge into pathogen free effluent and Class B biosolids.  It serves a population of 140,000 residents. Most of the 30,781 lbs./day of solid mass is used for landfill or applied to agricultural land as fertilizer.  The WWTP uses centrifuges to separate sewage. The resulting solids then undergo lime stabilization, creating a biosolid with low levels of pathogen and microbial activity. This effectively reduces the environmental and health risks associated with raw sewage. The treatment process, however, is expensive and does not fully remove hazardous pathogens. As the WWTP ages, the City must review alternative technologies in sewage treatment and determine the proper location to implement these changes.

There are many advantages to composting wastewater solids. The process reduces landfill space and creates a reusable, easy to handle product. Composted biosolids have agrarian, horticulture, and even environmental uses. Compost mimics wetland soil by pre¬venting the overloading of nitrogen, facilitating the absorption of ammonia, and blocking the transport of pollutants to adjacent surface waters. It can be used for wetland restoration projects such as protecting fragile ecosystems along the Huron and Saline Rivers and their surrounding tributaries.. Some municipalities are even using their compost to filter harmful chemicals from storm water runoff. 

There are however some potential drawbacks to composting biosolids. The process, if not well maintained, can produce bad odors and harbor pathogens. There is a risk of poor air quality at the composting center and neighboring land. Also, phosphorous and nitrogen rich runoff can negatively impact surrounding watersheds and ground water quality. It is important to pick a com¬posting site that minimizes these adverse risks by locating it away from densely populated areas and sensitive ecosystems (Figures 1-3) 

It was estimated that a capital cost of $25,188,988 would be needed to implement a belt filter press system into the current Ann Arbor WWTP. The BFP, on the other hand, would have relatively low electricity costs, being estimated at around 7,201 $/yr.  Currently, the WWTP spends 130,900 $/yr. on lime and 900,000 $/yr. on electricity. The higher initial investment cost would be offset by the significantly lower cost of operation while the sale of  compost produced from the BFP could be used to assuage the overhead and provide environmental benefits to the Ann Arbor community.