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Private treatment systems

Mitigation title
Private treatment systems
Description

Including systems like septic tanks and Package Treatment Plants (PTPs):

  • Septic tanks are chamber systems that hold raw sewage from dwellings for a short period of time. This allows for settling of the sludge and discharge of effluent treated by natural processes such as anaerobic digestion
  • Package treatment plants (PTPs) are similar in that they are also a chamber based system. However, aerobic bacterial biodegradation of the wastewater is encouraged by continually pumping air into the liquor. In comparison to septic tanks PTPs are able to provide a more highly treated effluent. The recommendation is to use PTPs over septic tanks or upgrade from septic tanks to PTPs if looking to achieve improvements in nutrient reductions. Removal rates is based on the system and certification can be provided by manufacturers

A further way to remove nutrients in private treatment systems is through dosing. This solution is typically undertaken at Wastewater Treatment Works (WwTW), but can also be undertaken at a smaller scale in package treatment plants (PTPs). It involves the addition of iron based compounds, which precipitate phosphorus. Phosphorus can then be removed either by settling or filtration.

Advantages
  • Water quality improvements dependent on system used/improvements
Disadvantages
  • Significant nutrient loadings risks to surface waters if discharges to ground are not managed or if the equipment is not maintained correctly
Parameters
  1. Phosphorus
  2. Ammonia
  3. Nitrogen
  4. BOD/COD
  5. Suspended solids
Carbon footprint
  • Low carbon footprint
  • Large footprint of iron supply for dosing
Time to become effective
  • Immediately
Maintenance
  • Annual maintenance required
Performance with time
  • Performance can be kept at optimum with well a well-established maintenance schedule
Scaling considerations

The size will be dependent on the population it has to serve.

References

Ricardo for Herefordshire Council. (2021). Interim Phosphate Delivery Plan Stage 2, Mitigation options for phosphate removal in the Wye Catchment.