The effects of wastewater treatment on wetlands in Cedarville, Michigan.

Abstract

Throughout history, people have sought ways to rid themselves of their waste products. Since the industrial revolution, humans have begun using and contaminating more water per capita than before. Therefore, in recent years, proper disposal of liquid wastes has become an increasingly pressing issue. With urbanization and increased population density came water treatment to purify water before its reintegration into the watershed. Conventional wastewater treatment plants employ a series of filtrations and chemical reactions to remove solids, pathogens, and typical household and commercial contaminants (Fernandez, 1997). Primary and secondary treatments remove solids and organic particles though filtration and anaerobic digestion (FAQ, 2001). First, influent is mixed and aerated, causing dissolved gasses to fall out of solution. The wastewater is then transferred to settling tanks filled with activated carbon. Active carbon binds with solids and small particulates, forming clay like sludge (Hammer and Viessman, 1985). Sludge is passed to anaerobic digesters where microbial action breaks down solid wastes. Liquid is siphoned off and sent back to the settling tank (Mitchell, 1972). Next, water flows from the settling tank through a gravity filtration process. Sand or diatomaceous earth removes some nutrients and chemicals. In tertiary wastewater treatment, chloride and fluoride additives remove excess nitrates and organics through anion exchange. These halogens also act as bactericides by oxidizing the cells of microorganisms (Hammer and Viessman, 1985; FAQ, 2001). This treated water is then released back into the water cycle. Wetlands are naturally sinks for many nutrients and chemicals (Mitch and Gosselink, 2000). Thus, the use of wetland ecosystems for tertiary filtration has become an economically viable and relatively popular alternative to conventional tertiary wastewater treatment (Kadlec,1979). After wastewater undergoes primary and secondary treatment, it is released into a wetland. As water flows through a wetland, plant roots act as filters, slowing the flow of water and trapping nutrients along with sediment. The plants absorb nutrients such nitrogen and phosphorous, incorporating them into their biomass. This locks up the nutrients, taking them out of the system and cleansing the water (Gopal, 1999; Kadlec, 1979). Additionally, organic soils adsorb phosphate (Kadlec, 1979). Nutrient loading, build up of heavy metals and halides, and noteworthy increase in the water budget contribute to change the ecosystem. This results in a system dominated by tolerant species (Gopal, 1999). These alterations in the wetland ecosystem can range from 'barely detectable to dramatic' (Kadlec, 1979). In this study, we sought to examine the effects of tertiary wastewater treatment on the costal marsh in Cedarville, Michigan. Since August 1996, Clark Township Sewage Treatment Plant has used the marsh surrounding Cedarville Bay for filtration of the city's wastewater (EPA 2001). Although the treatment plant completes primary and secondary treatment, phosphorous (P), chloride (Cl), and some nitrogen are not removed from the water. Secondary effluent flows from the plant on Blind Line Road in Cedarville, Michigan down Pearson Creek to Cedarville Bay. Large amounts of wastewater are released twice a year, once in the spring, before Memorial day, and once in the fall, after Labor day (Grant, 2001). Prior to 1996, Clark Township Treatment Plant sprayed effluent into the cedar swamp surrounding the treatment plant at the headwaters of Pearson Creek. This water was largely untreated and thus is presumed contain large amounts of nutrients (Grant, 2001). The control sites we used in our study were the costal marsh at the edge of Mackinac Bay, adjacent to M 134, and the conifer swamp across the highway. In exception to a parking lot, look out platform, and small picnic table by the marsh, the area is largely undeveloped and open to the public. Although the two sites have different disturbance histories, they are both costal marshes on Lake Huron in the Les Chaneaux area of Michigan? eastern upper peninsula.