Wastewater Treatment Facilities

There are 22 wastewater treatment facilities (WWTFs) in RI and MA that impact the waters of the Providence and Seekonk Rivers and Upper Narragansett Bay. (See map) All WWTFs are subject to NPDES permitting and they monitor and report discharges of permitted substances to the EPA's Permit Compliance System (PCS).  Discharges of nitrogen are reported as ammonia, nitrate, nitrite, or total nitrogen.  

• Monitoring data for the year 2000 and 2001

Wastewater treatment facilities discharge the most treatable part of the nutrient load entering Narragansett Bay. (RIDEM, WPWQ, 4.)  Nitrogen enters the domestic wastewater stream primarily as urea and combined in feces and other organic material. (WPCF, 6)  Ideally, wastewater undergoes two processes in the wastewater treatment facility: nitrification and denitrification.  A highly simplified explanation of these processes is as follows:

Nitrification:

Urea hydrolyzes to Ammonia (NH3)

Ammonia oxidizes to Nitrite (NO2-)

Nitrite oxidizes to Nitrate (NO3-)

Nitrate feeds algae

 

Denitrification:

Nitrate is reduced to Nitrite

Nitrite is reduced to Nitric Oxide (NO), Nitrous Oxide (N₂O) or Nitrogen Gas (N₂)

Click here for a beginner's guide to nitrification/denitrification technologies

The removal of nitrate from treated wastewater through denitrification would be an important step in reducing nutrient pollution in the receiving waters, as the process of nitrification produces fertilizer for algae.  Denitrification, however, is expensive and land intensive, and few facilities provide this treatment.  Still, it is especially important for facilities to provide nitrification.  Not only is ammonia toxic to fish, but if ammonia is allowed to enter the receiving water, it will be nitrified by nitrifying bacteria there.  This nitrification process is extremely oxygen intensive (using 4.2 mg of oxygen for each 1 mg of ammonia converted. (WPCF, 26)) and can result in an oxygen sag downstream of the facility.  When this process is complete, which can happen very quickly under the right (worst) conditions, the ammonia will have been transformed to nitrate, which fertilizes algae, which die and decompose, which further reduces dissolved oxygen. (WPCF, 16)  Therefore, the oxygen demand of discharged ammonia and nitrite must be counted twice -  once as it is nitrified - and then again when it produces biomass which must then be decomposed. 

• Analysis of monitoring data for the year 2000

Of the total nitrogen input to the Bay, 28% comes from direct discharges to the Bay from WWTFs, 5% comes from precipitation, 6% comes from runoff, and 62% comes from rivers and streams.  The contribution from rivers and streams includes discharges from upstream WWTFs. (Nixon, 15-61)  A draft update to these figures from Nixon estimates that when the nitrogen contribution to river flow from WWTFs is separated from other sources, the total contribution of nitrogen to the Upper Bay (north of Prudence Island) from WWTFs is 66%. (RIDEM, 2)