Municipal Wastewater Dosing

Chemical dosing in a municipal wastewater plant is concentrated in three places: chemical phosphorus removal after the biological stage (PAC, ferric sulfate or other iron/aluminium salts), coagulation and filtration in tertiary treatment (PAC and polymer), and effluent disinfection (sodium hypochlorite or chlorine dioxide). Many plants still run fixed dose rates set by experience, while influent flow and load swing with time of day, season and rainfall. A fixed rate either wastes chemical and produces extra sludge, or leaves phosphorus, suspended solids and coliforms at risk of exceeding limits.

The chemicals are also demanding on hardware. PAC and ferric salts are corrosive, hypochlorite off-gasses, and polymer solutions are viscous. Wrong head materials, unsuitable check valves or poor degassing lead to inaccurate dosing, gas locking and crystallised valve balls, and operators end up dismantling pumps far too often.

Our approach is flow-proportional dosing, corrected by online analyzers, delivered as integrated dosing skids, configured per process stage:

1. Phosphorus removal and coagulation: mechanical diaphragm metering pumps sized from the design dose with sensible margin, paced by a 4–20 mA signal from the influent flowmeter. An online turbidity or suspended-solids analyzer on the effluent lets the PLC trim the ratio and avoid overdosing.

2. Disinfection: hypochlorite pumps are interlocked with a free or total chlorine analyzer using flow feed-forward plus residual feedback. PVC or PVDF heads with degassing valves reduce gas locking.

3. Dosing skids: tank, mixer, duty/standby pumps, safety and back-pressure valves, pulsation damper, strainer and control panel are pre-assembled so the site only connects chemical, water, power and signals. Polymer lines can use an automatic powder make-up unit.

4. Operation: pump status, tank level and analyzer readings can be passed to plant SCADA, with low-level alarms and automatic standby changeover.

To size a system we need the design flow and peaking factor, chemical type and concentration, target dose, injection-point pressure, and the installation environment. Our engineers then select pump type, materials and skid layout, and can provide comparable options from different brands.

Practical notes: keep PAC and ferric lines short without high points that trap gas, and flush heads and lines before long shutdowns to prevent crystallisation. Store hypochlorite away from light and heat and use it within a reasonable time, since loss of available chlorine reduces the effective dose.