- 6.1 Pollution Potential of Two-Pond Effluent Discharge
- 6.2 Effluent Discharge Standards
- 6.3 Ammonia, Pathogens1 and Pesticide Residuals as Pollution Parameters
6.0 DSW pond effluent and the sustainability of environmental resources
6.1 Pollution Potential of Two-Pond Effluent Discharge
Treatment ponds target BOD reduction; reduction in other waste parameters (N&P, suspended solids, ammonia, pathogens) is incidental. The loading to the environment of several pollution parameters in DSW two-pond system effluent may be compared to that from human sewage ponds in order to provide a perspective on the scale of the management problem of pond-treated DSW.
Table 6
Pollution load from dairy shed wastewater two-pond system compared to pollution load from human sewage pond.
| Pollution Factor | Effect in receiving water | Load in DSW 2-pond effluent (g/cow/day) | Load in sewage pond effluent (g/person/day) | Human population equivalence (people/cow) | Sewage pond equivalence to effluent from 300-cow 2-pond system (# of people) |
| BOD5 (organic) | Dissolved oxygen | 4.8 | 4.5 | 1.1 | 320 |
| BOD5 (organic + ammonia nitrification) | Dissolved oxygen | 28.9 | 7.2 | 4 | 1,200 |
| Ammonia Nitrogen | Toxicity and algal growth | 5.6 | 0.56 | 10 | 3,000 |
| Total phosphorus | Algal growth | 1.8 | 0.8 | 2.25 | 675 |
Basis: Effluent from two-pond systems treating DSW from cows grazing high Nitrogen content herbage from fertilised pasture
6.2 Effluent Discharge Standards
In general, numeric standards for effluent discharge from two-pond systems have not been used by Regional Councils to control discharges of DSW. The two-pond system is specified as a treatment system by which Councils' requirements for DSW treatment prior to discharge will be met. The assessment of the acceptability of an effluent for discharge is that the effluent should come from a two-pond system that is properly designed, sited, constructed, and maintained. This approach has been accepted by the dairy farming industry.
Regional Councils in considering numerical standards for the future for two-pond effluent discharge are focusing mainly on the traditional parameters BOD5 and suspended solids. These standards are seen as measures of the performance of two-pond systems; they are not seen by Councils as being used in direct reference to sustainability of the resource values of receiving waters. An example of this is the standard of 150g/m3 for BOD, and suspended solids proposed by the Waikato Regional Council for its regional rules regarding dairy shed effluent. This standard was arrived at after investigations into what can reasonably be expected in effluent quality from properly sized, installed, and maintained two-pond systems. Notably this figure of 150 g/m3 was revised upward from an earlier proposed figure of 100 g/m3.
As the design basis for two-pond systems is BOD5 removal, and other pollution parameters (for receiving waters) such as nutrients, ammonia, pathogens, and pesticide residues are not specifically targeted by oxidation ponds, these other parameters are generally ignored in regard to effluent discharge standards, For no pollution parameter, including BOD5, is the impact on aquatic resource sustainability currently considered in a rational setting of a discharge standard.
Councils generally have insufficient information on the resource values of receiving waters and the impacts caused by discharge of treated DSW that they can at this stage take a rational, catchment-management approach to setting discharge conditions that relate to what is required for sustaining or enhancing the resource value receiving waters.
The message from council officers in principle dairying regions is that Regional Councils are moving or intend to move toward a more receiving water based, catchment approach to controlling DSW discharges. In developing this approach, attention is being given to other treatment technologies, other disposal options, and the economic impacts on dairy farming businesses and the community at large.
In the consideration of other treatment technologies and possible requirements for treatment beyond what is achieved by two-pond systems, we suggest that ammonia (nitrification) oxygen demand, pathogens, phosphorus and possibly pesticide residuals need more to be addressed than further reduction in BOD5, and suspended solids in the effluent for discharge.
6.3 Ammonia, Pathogens1 and Pesticide Residuals as Pollution Parameters
6.3.1 Ammonia
BOD5 is taken as measured by a standard BOD test, which does not measure the BOD5 of ammonia oxidation (nitrification). The BOD5 of ammonia is about four times the concentration of ammonia-nitrogen in an effluent. In the case of sewage oxidation pond effluent, the true BOD5 of the effluent may not be very much greater the measured BOD5 as ammonia in sewage pond effluent is usually at a low level. This is not the case with DSW two-pond effluent where the true BOD5 (nitrification oxygen demand plus the measured (i.e., organic) BOD5 may be five times the measured BOD5.
The other and perhaps more important concern regarding ammonia in DSW pond effluent is ammonia toxicity to aquatic animals. This will be of greater concern in "cleaner" streams in which Mayfly larvae which, by their dominating numbers and production in such a stream, may have an important structuring influence on the stream ecosystem. Mayfly larvae are particularly sensitive to ammonia. For a stream with an ecosystem featuring animals sensitive to ammonia, the dilution requirements for safe discharge of dairy shed pond effluent to a stream may be several times greater for ammonia than for other effluent parameters (Hickey et al, 1989)
6.3.2 Pathogens
Reduction in the level of pathogen indicators over two-pond systems is not great. Effluent faecal coliforms concentrations in the order of l04 to 105 are typical. In instances where it is determined by a Regional Council that bathing water quality standards on even stock water quality standards shall be maintained in a small waterway, discharges of two-pond effluent have significant potential for causing these standards to be breached.
6.3.4 Pesticides and Anthelmentics
There is no information that we are aware of regarding pesticide residues in DSW pond effluent. It is probable that such residues occur in pond effluent and it would seem prudent that potential impacts resulting discharge or land application of pond effluent should be investigated. This would be initially a simple desk-top study.
6.4 Greenhouse Gases
In ambient temperature anaerobic ponds. methane production of about 0,2 m3 CH4 per kg volatile solids loaded to the pond is widely reported. This translates to about 20 m3 of methane per day from an anaerobic pond serving a 300 cow herd, and represents about 4.4% of the methane emitted by the cows in a day
Some lesser and variable amount of CH4 is produced when manure is deposited straight onto the ground by cows.
Nitrous oxide is also released to the atmosphere when wastewater containing ammonia is irrigated to pasture. This is a result of nitrification of the ammonia to nitrite and/or nitrate followed by incomplete denitrification (complete denitrification produces nitrogen gas). Again, this occurs also when manure and urine is deposited by the cows onto land. Nitrous oxide is a potent greenhouse gas and the generation of it from land application of DSW may be an issue for caution in laud application.
In summary any form of management of dairy shed waste for the purpose of reducing the greenhouse gases arising from this waste will not achieve significant reductions relative to the total greenhouse gas emissions attributable to cows.
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