Nearshore marine waters and estuaries are deteriorating globally, particularly from nitrogen pollution that leads to severe hyper-eutrophication causing a cascade of adverse impacts to ecosystem function, including critical habitat disruption and fishkills from hypoxia. Nitrogen is the limiting factor in marine waters of which excessive amounts lead to these problems.
A dynamic strategy is being developed and demonstrated on Cape Cod in Massachusetts to address this problem in a cost-effective, energy efficient and resource conserving manner that minimizes capital and operating expense and climate impacts. The strategy prioritizes the use of commercial shellfish aquaculture and nutrient capture and reuse via urine diversion (UD), collection and application as agricultural fertilizer. Urine contains 80% of the nitrogen in human waste or about 3 kg-N/person/yr in the USA, based on research conducted by The Green Center in conjunction with the Massachusetts Septic System Test Center. Urine is generally free of contaminants and is pasteurized prior to application to crops such as hay fields or vegetables. As the cost of chemical fertilizer has escalated, urine has become a more desired alternative.
In the case of shellfish aquaculture in estuaries, the nitrogen Total Maximum Daily Load is first determined for a healthy ecosystem followed by a n assessment of the estuary’s carrying capacities – allowable area, production capacity, ecosystem sustainability and social acceptance -- for shellfish aquaculture. The eastern oyster, Crassostrea virginica, is the preferred shellfish due to its rapid growth and marketability. At harvest size of 7.5 cm, one contains 0.28 g-N, per findings on Cape Cod or can assimilate about 700 kg-N/ha/yr. Therefore, for example, if 1,000 kg-N/yr needs to be removed from an estuary to meet its nitrogen TMDL, in addition to one hectare of oyster culture, about 100 people would be needed to adopt UD technologies in its watershed. UD fixtures and plumbing needs are easily adopted in most homes and are much lower cost than that of sewer infrastructure both in terms of construction and operation needs.
In Falmouth, Massachusetts with 33,000 year-around residents, a three-year, pilot shellfish aquaculture program was recently completed via a public-private partnership by the Town with three growers, using 0.6 ha in one estuary. In 2025, the Town next plans to expand to 6 ha in a few of its 15 estuaries all with degraded water quality due primarily to septic effluent. The Town is also obtaining permits from the state and designing a UD pilot project to start in 2025. This ecological, cost-effective, climate friendly hybrid approach has enormous promise.