1.0 Introduction
1.1 Synopsis
Treatment ponds for stabilisation of organic wastewater arc a long established low cost, simple and effective pollution control technology. They are particularly appropriate in rural areas. Two-pond systems - an anaerobic pond following by an aerobic pond were introduced for treatment of dairy shed wastewater on New Zealand farms in the 1970's. Being regarded as the best practical option for treating dairy shed wastewater prior to discharge, these pond systems now number in their thousands and are the most common method of treating dairy shed wastewater.
In response to concerns expressed about the effectiveness and role of two-pond systems for treatment of dairy shed wastewater, the Policy Section of the Ministry of Agriculture and Fisheries commissioned a report to examine these concerns in terms of:
- The treatment processes of oxidation ponds n general arid the design basis of two-pond Systems for dairy shed wastewater.
- Performance expectations and actual performance of two-pond systems.
- Analysis of functioning of ponds treating dairy shed wastewater.
- Design modifications for performance improvements.
- Two-pond effluent and the receiving environment.
- Regional Councils and management of the use or two-pond systems.
- Appropriate role of two-pond systems and other treatment systems in the management of dairy shed wastewater.
This report also gives recommendations and information to assist the industry and Regional Councils in moving towards appropriate use of a broader range of treatment/disposal technology (including two-pond systems) for dairy shed wastewater.
1.2 Background
In New Zealand, dairy herds typically graze pasture for most of the year. Cows are confined only during milking operations - a total time of from 1.5 to 3 hours per day The controllable manure waste from NZ dairy herds comes from this short period of daily confinement, and comprises from 6 to 12 percent of total daily production of manure.
Manure from dairy shed confinement is flushed away with water, and this, along with udder washwater, milking plant washwater, and any milk spillage constitute Dairy Shed Wastewater (DSW).
The pollution potential of a wastewater depends on the receiving environment and the manner of treatment and disposal, and the amount of waste produced. Table 1 shows typical amounts of several parameters of waste produced per cow in dairy shed wastewater for three values of average time spent in the holding pond.
The concentration (strength) of DSW also is very variable and will depend on the waste load and the amount of water used for flushing the pad. Waste load generally varies during the milking season. Flushing volume does not vary directly with load fur an individual shed and holding pad; however, wet, muddy weather may cause more water to be needed for flushing.
Table 1
Per cow production of waste in dairy shed wastewater for 3 average times spent on the holding pad. (values calculated for 450kg cow from manure production figures from Table 2.3 in the Agricultural Waste manual (NZAEI, 1984); milk spillage would increase these values.) Values from the AWAS Table 2.2 are also shown.
Waste Load in DSW (g/cow/day)
| 60 mm on Pad | 120 mm on Pad | 180 mm on Pad | AWM Table 2.2 | |
| Total Nitrogen | 9 | 18 | 27 | 10 |
| Total Phosphorus | 0.9 | 1.9 | 2.8 | 1.8 |
| BOD5 | 37 | 74 | 110 | 80 |
| Total Solids | 165 | 330 | 495 | 360 |
| Volatile Solids | 120 | 240 | 360 | 250 |
Until the late seventies, most dairy farms in New Zealand discharged DSW untreated into the nearest waterway - often a farm drainage ditch, sometimes a natural waterway. Surface water quality in areas of intensive was considerably degraded (e.g. Taranaki Region), and catchment authorities (now Regional Councils) for the past 10-20 years have encouraged the installation of two-pond oxidation pond systems to treat DSW prior to discharge.
Two-pond systems (described below) were considered to be the best practical option for DSW treatment -suggested achievable BOD, and 35 removal was 90-95% (NZAEI, 1984), the technology was cheap to install, and the operational requirements (virtually none) and maintenance requirements (minimal) were at a level that farmers could accept.
However, reports on the performance in practice a two-pond systems highlighted that effluent quality was very variable and often poor (Rangitikei-Wanganui Catchment Board & Regional Water Board, 1984; Hickey et al, 1989; Taranaki Regional Council, (1990). Deficiencies in the installation and management of two-pond systems were common, hut no explanation was apparent for the high variation in effluent quality apparently correctly sized and installed two-pond systems.
Further, there has been increasing concern about the impact of other pollutants in DSW (ammonia, phosphorus, pathogens, etc.), and the efficacy of two-pond systems in reducing the amounts of them in the effluent.
The Waste Technology Group of MAF-Invermay advised of concerns about the design fundamentals of two-pond systems and the propensity for some Councils to require farmers to install two-pond systems based on MAF guidelines and at the same time specify an effluent discharge standard unlikely to be consistently achieved by systems so-designed. On the basis of these concerns MAF in 1991, withdrew the MAF Aglink guidelines on drain shed effluent ponds.
The dairy farming industry as well as Regional Councils have accepted that in some cases treatment by two-pond systems Followed by discharge does not constitute management of dairy shed effluent that is sustainable under the Resource Management Act. (Land application of raw or treated DSW in a manner designed to achieve Slow Rate Land Treatment is now considered by many Councils to be the BPO for DSW management, thereby avoiding risks of environmental damage to waterways and at the same time accruing benefits to the land.)
1.3 Brief for Policy Research for MAF Policy
The Policy Section of Ministry of Agriculture -tool Fisheries, concerned about the use and status of two-pond systems as the BPO fur treatment, commissioned the Waste Technology Group of AgResearch (now Waste Solutions Limited) to provide a report addressing three objectives:
- Description and analysis of the performance of farm ponds with respect to current design standards and actual installation and operational practice.
- An assessment of the feasibility of improving the design and operation of pond systems to meet higher standards of pollution control.
- A review of the perceptions and activities of Regional Councils in dealing with the issue of dairy shed waste management by pond systems.
1.4 Methods
Key dairying areas of New Zealand (Southland, Waikato, Taranaki, and Northland) were visited. Regional Council officers involved in the administration, inspection, and performance-monitoring of dairy farm two-pond systems were consulted for information regarding all the objectives above.
Objective one was addressed by studying available technical literature on two-pond systems and further by studying a number of pond-systems (5-10) in each area that were reputedly in a good operational state, i.e., not due for de-sludging. These ponds were inspected with regard to:
- Loading rates (sizing)
- Engineering aspects of the ponds (i.e., pond geometry and type of inlet and outlet structures)
- Performance (i.e., effluent quality)
Additional functional parameters of some ponds were examined:
- Settleability and BOD5 of suspended particulates at several points in ponds systems
- Phytopigment concentrations and attenuation of Photosynthetic Apparent Radiation in 2nd ("aerobic") ponds
- Oxygen availability and mixing as indicated by DO and temperature profiles.
Objective two was addressed by relating the function and processes of two-pond systems to established engineering principles for optimally achieving these functions and processes.
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