Low-Velocity Force Main and Crown Corrosion
Your 250 mm force main is 2.1 km long and carries sewage from one lift station to a gravity manhole. Average flow is 28 L/s; when the pump runs, it delivers 45 L/s. The pump runs about 15 hours per day and is off the rest. At the discharge manhole, H2S reads 40 ppm and the concrete crown is soft and exposed. Residents report odour. The force main has no air-release valves. A chemical supplier proposes sodium nitrate dosing at the pump station. Your capital budget is limited, and the pipe cannot be replaced this year.
1. Calculate velocity and detention time at average flow and at pumped flow. What do these values tell you about solids deposition and sulfide generation?
Read the worked answer
Area = π(0.125)^2 = 0.0491 m².
At 28 L/s: v = 0.028/0.0491 = 0.57 m/s; detention = (0.0491 × 2100)/0.028 = 3680 s = 61 min.
At 45 L/s: v = 0.045/0.0491 = 0.92 m/s; detention = 2290 s = 38 min.
Average velocity is below the 0.6 m/s scouring target, and detention is long. Because the pump is off about 9 h/day, actual stagnant time is greater than the average detention. Low velocity, long stagnation, and no air valves create anaerobic conditions and sulfide. The 40 ppm H2S and soft crown confirm corrosion.
2. Rank the options: sodium nitrate dosing, increasing pump run time, adding air-release valves, flushing, and pipe replacement. Justify short-term and long-term choices.
Read the worked answer
Short term: sodium nitrate dosing is the most practical immediate control because it suppresses sulfide in the force main and can be started without excavation. It must be dosed and monitored, not just purchased. Increase pumping frequency only if it shortens stagnant periods without lowering velocity; simply running longer at 45 L/s is not possible unless inflow increases. Air-release valves help air pockets and surge but will not fix low velocity or sulfide. Periodic flushing is a temporary measure.
Long term: replace or reline the force main, consider a smaller-diameter or parallel main to raise velocity, and add corrosion protection. Rank nitrate dosing first for this year, with flushing and odour control; plan capital replacement next.
3. What monitoring and safety controls would you require for nitrate dosing and for downstream manhole work?
Read the worked answer
For nitrate dosing: contain storage, control dose to avoid overdosing, monitor nitrate/nitrite, DO, and H2S at the discharge, and train staff on the SDS. Nitrate can create its own permit issues and is hazardous in bulk.
For downstream manhole work: the H2S source may still be present; use confined-space permit, continuous monitoring, ventilation, retrieval, and attendant. Because the crown is soft, avoid knocking off concrete and do not enter without assessing structural condition. Notify residents of odour and any advisory. Keep records of dose, readings, and maintenance.