Summary
The environmental pollution potential of dairy shed wastewater is considerably reduced by the use of two-pond oxidation pond systems for treatment. Two-pond systems have been seen as a general Best Practical Option for treating dairy shed wastewater prior to discharge.
The proportion of dairy farms using two-pond systems has dramatically increased over the past fifteen years. Offsetting this improvement in effluent quality is the large increase in numbers of dairy farms in some regions.
The potential pollution load of two-pond treated dairy shed wastewater is equivalent to that front a facultative sewage pond serving a human population of from one to four times the size of the dairy herd, depending on the pollution parameter being considered.
Currently, Regional Councils use regional rules, inspections, enforcement procedures, and monitoring to manage the design, installation, and maintenance of two- pond systems. This requires considerable commitment of Councils' resources in order that the incidence of two-pond system dysfunction through neglected maintenance or non-compliance with regional rules he kept at a low level.
In reducing the impact of dairy shed wastewater on receiving waters, there has been a focus on reducing the amount of BOD5 and suspended solids in the effluent. Evidence in this report shows that the efficiency of two-pond systems in doing this is less than has been supposed.
Other pollutants in dairy shed wastewater, eg, ammonia, phosphorus, and pathogens, are nor targeted for treatment in two-pond systems design. Although a significant reduction in ammonia is achieved, the ammonia levels remaining in the effluent require a high dilution rate for safe discharge to sensitive aquatic ecosystems.
Because two-pond systems and barrier ditches have been the only technologies to have Best Practical Option status for treatment for discharge, contaminants other than BOD and suspended solids in dairy shed wastewater have not been, till now of practical concern to Regional Councils. Councils are moving towards managing effluent discharges in regard to ammonia and pathogens.
Land disposal of treated or untreated dairy shed wastewater is encouraged by Regional Councils as a treatment/disposable-use solution that, when well-managed, avoids contamination of surface waterways. Because of the constraints and problems that can arise in achieving effective and safe land application or dairy shed wastewater, land application is not suitable for all farms and is not a panacea for management of surface water quality. Two-pond systems remain the principle accepted technology for treatment prior to discharge to surface waterways.
Changes in procedures for estimating waste loads from dairy sheds, and changes to the sizing and design of ponds, could result in a reduction in the high-end variance in pond performance in regard to BOD reduction, and allow reduction in the ammonia content of the effluent to be a treatment target. Such changes are at the concept stage at present and would require development and pilot trials before changes could be generally recommended. Preliminary suggestions are made in this report for design changes that should improve the performance of two-pond systems such that effluent BOD5 reliably less than 100g/m3 and ammonia -N less than 70g/m3 maybe expected.
Now that dairy farmers have accepted two-pond systems for treatment of dairy shed wastewater, they are reluctant to embrace other technology that may cost more or may require a greater commitment to operation and maintenance. Further, there is a feeling that pollution control management of dairy' shed wastewater should be a once-and-for-all effort/expense for the farmer. These attitudes need to change if Regional Councils seek to improve over time the quality of aquatic natural resources in areas where the quality has diminished over the years. In this regard, dairy shed wastewater management should not be addressed in isolation from other aspects of dairy farm management including the scale and siting of the dairy farming industry in some areas.
There is a need for investigation and development of treatment/disposal technologies for use where two-pond systems provide insufficient treatment for discharge to surface water and where land application for slow rate land treatment is not a practical option. Reduction of ammonia and pathogens could become the main targets of treatment as Regional Councils seek through their regional plans and implementation programmes to improve surface water quality. Technologies to consider are total containment ponds for zero or occasional discharge, overland flow treatment systems, wetlands systems, and infiltration basins for pathogen reduction, intermittent cycle mechanically aerated ponds for high efficiency removal of BOD, suspended solids, and ammonia. Desktop review of these technologies is an essential first step in their investigation for possible on-farm use.
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