Struvite buildup in recycled effluent systems
Recycling effluent for yard washing reduces water use and storage pond sizing, but can have disadvantages with build up of salts.
A number of farmers in the Colac district have reported poor pump performance because of a crystalline buildup on the pump fittings and pipes. This has occurred when recycling continuously over dry seasons due to water shortages.
Overseas experience shows that the crystals are the chemical Struvite, which is a magnesium ammonium phosphate. This is the same chemical that causes kidney stones.
In dairy effluent recycling systems struvite needs to be kept from moving out and causing pumping problems.
Why the crystals form
With continued recycling the salts in the effluent accumulate and become supersaturated and if conditions are right precipitation occurs.
The main factors to speed up precipitation are:
- pH7 (normal effluent level)
- rough interior pipe surfaces
- metal components
The crystals stay in suspension in acid conditions (pH4) but the effluent pond will not work properly at this pH level. Plastic components are not immune to build up but are less attacked than metals. Turbulence is affected by:
- pump size
- pipe size
Crystals accumulate where there is turbulence in the water flow so this means around valves, joins, bends and the pump.
Minimise struvite buildup
- The first thing is to dilute the effluent to reduce the chances of salt accumulation. In the short-term use fresh water instead of recycled water. This may be enough to stop the problem getting worse but won't fix it. During drought when water is in short supply it may not be practical.
- Make sure the recycling pond is emptied out each year. This means in a one-pond system, the pond should be emptied yearly, and the second of a two-pond system should also be emptied yearly. These are recommended measures anyway to ensure ponds do not overflow.
- Check salt levels in the pond and try not to exceed 3000 to 5000uS/cm. This will be difficult with some bore waters (some of which normally have high levels of magnesium in the western district). With bore water some of the salts maybe harmful in excess and care is needed with application rates and soil types to avoid soil degradation problems. This emphasises the above point that ponds should be emptied yearly.
- When recycling check system to reduce turbulence. Pump and pipes need to be matched. If too large a pump is used then high flow rates and turbulence follows. Size pipes to keep velocity to 1 to 1.5m/sec. Minimum pipe size recommended is 50mm.
- Minimise suction lift by having pump near water level. Make sure suction pipe is large enough to prevent cavitation. Make it the next size up from the delivery pipe. Have gentle bends rather than sharp turns in pipes.
- Replace pipes and fittings. This may be easier than trying to dislodge the built up crystals, which need a hammer and chisel to break them off fittings. This has been the method used by most Colac district farmers.
- Acid cleaning solutions can be used to dissolve crystals. A recirculating system is best but care is needed if metal components are involved because the acid can corrode them. Care in handling acids is also needed. Follow all safety precautions. Hydrochloric acid used for cleaning bricks is diluted 1 part to 9 parts water and can be used to soak components or as a recirculating system. If recirculated the acid can be reused, but in time its strength will diminish. Disposing of acid also needs care.
- Make sure pump is grounded well to earth to ensure no stray voltage or electrostatic charges can contribute to crystal build up.