8.0 Ponds and other technology for appropriate treatment of DSW prior to discharge or land application
8.1 Rationale for Treatment
In most situations where discharge of two-pond effluent is deemed undesirable or unacceptable, then rather than treatment to a higher standard prior to discharge, land application of DSW (either raw or after pond treatment) is generally encouraged by Councils as a preferable alternative. Properly managed land application of DSW can accomplish beneficial resource cycling and a high level of renovation of the wastewater by Slow Rate Land Treatment before it joins the natural water regime.
However, treatment to a standard beyond what is achievable by two-pond systems has relevance for both discharge and land disposal. A higher level of treatment may be appropriate in order to:
- Reduce the pollution impact of discharge to a surface waterway where discharge is selected as the most appropriate option for disposal,
- Enable land disposal at higher hydraulic loading rates (e.g. disposal by rapid infiltration or overland flow) without causing groundwater contamination, excessive N loading or surface water contamination by run-off.
Treatment means reducing the pollution potential at the wastewater by reducing the amount of the polluting elements of the waste, The nature and extent of treatment required before disposal should depend in any instance on what mass loading- of pollutant(s) to the environment is permissible, in terms of the expected environmental impacts, for the particular disposal method being considered.
8.2 Passive Treatment Systems (Pond and Wetlands) for Enhanced Treatment
In keeping with the feeling that "passive" treatment systems are much to be preferred for use on farms, Regional Councils have been looking to constructed wetlands as an add-on treatment to two-pond systems for achieving further treatment. Pilot scale trials of wetlands for further treatment were set up by the Waikato Regional Council and the Bay of Plenty Regional Council.
Reduction of the amount of ammonia and pathogens in the effluent are seen as a principle objectives of further treatment by wetland, Removal of 75% of the ammonia nitrogen from two-pond effluent would require a wetland of about 1000m2 for a 300 cow herd. A wetland of this area would also provide significant removal of pathogens. During dry periods in summer when evaporation and evapotranspiration are high, a two-pond system followed by a wetland would in many instances be non-discharging.
Consideration must be given to how practical wetlands really are. The pilot-scale trials with surface and subsurface flow wetlands carried out by the Waikato Regional Council revealed major problems in establishing and maintaining the wetland plants. The Council's conclusion (not yet reported) was that the expense and difficulty in installing, establishing, and operating and maintaining a planted wetland cast severe doubts on their appropriateness as a treatment technology for use on farms.
Floating plant wetlands are one type of surface flow wetland that maybe viable for farm use for treating DSW floating plant wetlands using Lemna (duckweed) are easy to establish and are easily managed. Duckweed wetlands for treating DSW anaerobic pond effluent are well enough researched and developed to enable preliminary design guidelines to be drawn up.
Extending the area of "aerobic" pond in DSW pond systems can achieve better treatment for all parameters than conventionally sized two-pond systems. There is a danger, however, that lower loaded "aerobic" ponds will support extensive algal growth which, if discharged in the effluent, will increase the BOD load in the effluent. Consideration should be given to dividing an increased "aerobic" pond area into two ponds, with Lemna (duckweed) grown on the final pond to preclude algal growth by shading and to facilitate better fine-solids sedimentation by inhibiting convection mixing.
There is a limit to what level of further treatment can be achieved by increasing the area of passive treatment systems (ponds and wetlands) before the level of treatment sought requires an area of pond or wetland that gives total evaporation of the wastewater. The concept of disposal by evaporation, i.e., a total containment basin, should then be considered.
Overland flow systems for further treatment of DSW anaerobic pond effluent have not yet been investigated or developed. Such systems could be easier to install than wetlands, would occupy an area of land larger than wetlands but smaller than a irrigation system designed on a nitrogen loading criterion, and could be very effective for reducing the pathogens and ammonia content in the wastewater. They would not be a universally applicable option, as suitable sites must have the right soil permeability, slope, and drainage.
8.3 Other Treatment Systems
"Technical" treatment systems theoretically can achieve high treatment standards without the large area requirements of "passive" treatment systems. Because of the high proportion of slowly-and non-biodegradable solids in dairy shed waste, initial solids separation from the wastewater and separate treatment of those solids is a sensible first step of treatment. Anaerobic ponds are without doubt the most practical way of doing this on farms.
Technical refinements to anaerobic ponds to enhance digestion and gas production, would require mixing and possibly heating mechanisms. The benefits would be more efficient production of biogas and slower accumulation of sludge solids. The efficiency of ponds for methane production is in the order of 0.02m3 methane per m3 of pond per day A pond serving a 300 cow herd would produce about 20 m3 of methane per day. The value of this gas is $10.20 replacing LPG or $7.18 replacing CNG.
The costs associated with collecting and using the biogas (for example,. water heating), are greatly in excess of the return - covering a pond to collect gas costs in the order of $15,000.
For further treatment of anaerobic pond effluent, aerobic biological treatment is the most obvious approach. Standard technology for aerobic treatment includes trickling filters, rotating biological contactors, aerated backwash filters, aerated lagoons, and activated sludge. At present, none of these technologies could be considered feasible for use on farms, although, as discussed below, activated sludge systems hold the most promise for development to "practical technology" status for dairy farms.
Two of the principle goals of further treatment would be removal of ammonia and disinfection. Disinfection requires a high level of BOD and suspended solids removal before disinfection. Of the common technologies for aerobic treatment, extended aeration activated sludge is the cheapest and most practical for achieving high levels of BOD, suspended solids, and ammonia removal.
Although traditional operation of activated sludge processes requires a treatment plant and an operational effort far beyond what is appropriate for farms, we believe that activated sludge plants designed for intermittent cycle operation could be used for settled DSW. Such plants would take the form of a mechanically aerated pond with simple timer control of aeration, settling periods, and decant/discharge. We believe that the installed cost of an intermittent cycle extended aeration activated sludge pond could be in the order of $30,000 for a 300-600 cow herd, and that the operation and maintenance requirements would be less than for an irrigation system.
A benefit of a "technical" treatment plant, where water loss is minimised compared to a passive treatment system, is that treated DSW could be recycled for use as flushing water (Figure 2). By recycling, the volume of discharge to receiving waters could be reduced by about two-thirds, requiring less dilution. Land disposal also would require a smaller system than would otherwise be needed.
Disinfection of treated dairy shed wastewater is problematic because of the high amount of dissolved, non-biodegradable organic material staining the effluent after treatment. Ultra-violet disinfection is not effective for an effluent containing UV- absorbing dissolved organic material, and chlorination is a dubious practice for effluents containing dissolved organic material and is impractical for effluents containing ammonia. Wetlands overland flow, and sand filtration are possibly appropriate technologies for disinfection of DSW. Disinfection in an area of DSW treatment requiring investigation and technology development.
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