Dealing with SIN: A Novel Solution

The discharge of soluble inorganic nitrogen (SIN) to the Ōroua River from the Feilding Wastewater Treatment Plant (WWTP) has caused the Manawatū District Council (MDC) many headaches over the years. However, shifting from river discharge to land application of treated effluent triggered the law of unintended consequences; for the WWTP, which does not remove enough SIN for discharge to river, removes too much for land irrigation. This left MDC in the unenviable position of having to fertilise the crops irrigated with treated effluent.

A significant portion of the organic and nitrogen load to the WWTP is from trade waste contributors. Unlike a traditional nutrient removal WWTP, the Feilding WWTP has an anaerobic lagoon at the front end, which transforms a portion of this nitrogen into inorganic ammonia. Subsequent ponds with cycling aeration nitrify this ammonia but have proved unsuccessful in also denitrifying, further exacerbating the SIN problem.

The existing anaerobic digester reaching the end of its design life provided a timely opportunity for reconsidering overall operation of the WWTP. By diverting the high-strength trade wastes from the WWTP to the anaerobic digester for codigestion with the secondary sludge, thus creating a Waste-to-Energy facility, which will achieve the following four outcomes:

This paper will present the challenges and the opportunities for an inland rural community with substantial trade waste contributors. It will discuss the risks and benefits of dealing with SIN by codigesting trade wastes with WAS to boost energy yield.