Alternatives to control wastewater odors on board Washington State ferries and during discharge of the wastewater to sewerage systems were formulated, evaluated and tested.
Holding tank aeration, using compressed air injection to a recycle line is an effective method. Sulfide is microbially oxidized, and anaerobic sulfate reducing bacteria are suppressed. Engineering and lab studies and a simulation model of the process were used to estimate suitable design parameters and costs. A prototype installation is recommended for optimization of operation.
The addition of slug dosages of H202 was also found to be effective in laboratory and on-board vessels. The twice daily dose of about 50 mg/1 should be stirred into the holding tank using recycle mixing. Peroxide was found to oxidize sulfide, increase dissolved oxygen and suppress sulfate reducing bacteria. Bottom suction sludge hoppers must be installed to prevent accumulation of sludge in tank bottoms.
Other alternatives, including chlorine addition, pH increase, iron sulfide precipitation and toxic odor control compounds, were found ineffective and/or uneconomic.
Other finds are that sulfide is produced primarily from reduction of sulfates from the saltwater flushing water. Production is primarily in the slime layer on the bottom and sides of the holding tanks. On-shore force mains are sites of sulfide production during stagnant periods between tank pumpouts. These force mains should be flushed with clean salt water once each day. The discharge from ferry holding tanks was evaluated for hydraulic, organic loading, and salt toxicity effects on existing and planned sewage plants. No significant impacts were found.