The objectives of the project as set out in the MAFF contract are quite clear. For impermeable soils:
(A) To determine the ability of overland flow to reduce concentrations of BOD, MRP, nitrate- and ammoniacal-N following applications of dirty water under different soil conditions.
(B) To determine the influence of slope angle on reducing concentrations of BOD, MRP, nitrate- and ammoniacal-N following applications of dirty water.
(C) To determine the maximum loading rates of dirty water for different slope angles.
(D) To determine the influence of application frequency on purification efficiencies of BOD, MRP, nitrate- and ammoniacal-N.
(E) To optimise the field based soil treatment systems using the information gathered from the above.
Part of the desk study objective list is as follows:
(F) To map out areas where dairy farms can use existing suitable soil types for percolation based treatment systems and overland flow treatment systems.
(G) To consider suitability of the drainage water from such systems for discharge into watercourses, re-irrigation to grassland, possible uses in horticultural or fishery industries.
(G) To consider potential land-use implications of dedicated areas of land for soil-based treatment systems - e.g. potential life times of such systems and whether rotation is feasible. What to do with potentially contaminated land at the end of a system's life.
Throughout this document, "dirty water" is taken as that defined in the MAFF Code of Good Agricultural Practice For the Protection of Water (Welsh Office Agriculture Department, July 1991):
"Dirty water is waste, generally less than 3% dry matter, made up of water contaminated by manure, urine, crop seepage, milk, other dairy products or cleaning materials."
From the point of view of the science, and for final judgement of applicability to particular sites, it would be beneficial to determine, as far as is possible within the scope of the project, the processes that govern the treatment capacity of a treatment plane. It would undoubtedly prove easier to have confidence in the efficacy of overland flow, and to determine its limitations as a treatment method for dirty water if it was known why and how it worked.
Before commencing experimental work using dirty water on the treatment planes, it will be necessary to fully characterise both the dirty water and the soil onto which it will be applied. There are several reasons for this. Firstly, it would be beneficial, from an experimental standpoint in terms of consistency and in terms of convenience, to be able to standardise the applied waste. As the study is looking at differing soil conditions, it is sensible to keep the applied water as uniform as possible. This would be best accomplished by formulating a synthetic dirty water, perhaps by fluidising manure with the addition of water and adding urea to make up any ammonia deficit. To determine whether this is feasible, the chemical nature and composition of dirty water in terms of treatment demands must be determined as fully as possible, although it is understood that a certain amount of variability is to be expected. This will involve determination of the phosphorous content, the nitrate- and ammoniacal-N content and BOD in order to comply with the study objectives. To aid completion of the desk top study objectives, and for later determination of processes, it would also be beneficial to determine the concentration of selected metals and possibly identify any microbial components which may be of concern. Common diseases associated with contaminated irrigation waters include cholera, typhoid, ascariasis, amoebiasis, giardiasis, cryptosporidiosis and enteroinvasive E.coli [FAO 1996]. It is most likely outside the bounds of this study to determine the risk from pathogenic viruses in dirty water, but some bacterial work should be possible. A variety of metals have been found by researchers in studies (Tables 1a & 1b), and these may have significance both for land use after the end of a system's life, and for removal of the contaminants under consideration.
The soil will require characterisation, in order to determine the likely physico-chemical processes taking place on the treatment planes during the experimental work, and to aid in determination of the range of flow rates to use in order to determine the limits of treatment capacity. The objectives include a requirement to determine maximum loading, but this can only be determined, in this instance, for the soil in use, and it will be necessary to define that soil in order to make comparisons with other soils. To this end, it will be useful to determine phosphorous content, nitrogen content, organic matter content, phosphorous fixation capacity, content of selected metals for comparison with the dirty water findings, infiltration capacity, permeability and to formulate a water release characteristic curve. The nature of the construction work already carried out on the field site has meant that there is a quantity of plastic sheeting fragments in the soil on the treatment planes. For the sake of completion, the percentage volume will be estimated.
| Batch No. | 1 | 2 | 3 |
| PH | 7.49 | 7.73 | 7.70 |
| Dry Matter (%w/w) | 5.462 | 4.181 | 5.226 |
| Organic Matter | 4.328 | 3.085 | 4.007 |
| BOD5 (mg/kg) | 4128 | 2096 | 1672 |
| Total N (%) | 0.301 | 0.299 | 0.327 |
| Org N (%) | 0.140 | 0.145 | 0.150 |
| NH3 -N(%) | 0.161 | 0.154 | 0.177 |
| Ash (%) | 1.134 | 1.096 | 1.219 |
| Total P (%) | 0.051 | 0.044 | 0.058 |
| Soluble P (%) | 0.0022 | 0.0010 | 0.0016 |
| Total K (%) | 0.296 | 0.256 | 0.305 |
| Soluble K (%) | 0.134 | 0.155 | 0.161 |
| Ca (%) | 0.113 | 0.095 | 0.109 |
| Mg (%) | 0.041 | 0.045 | 0.047 |
| Chloride (%) | 0.100 | 0.108 | 0.101 |
| Na (%) | 0.065 | 0.070 | 0.065 |
| CO3 (%) | 0.042 | 0.037 | 0.040 |
| SO4 (%) | 0.052 | 0.046 | 0.054 |
| Cu (ppm) | 6.22 | 7.80 | 4.40 |
| Zn (ppm) | 14.80 | 11.28 | 10.60 |
Table 1a: Cattle slurry analysis (from Spallacci & Boschi, 1979)
| No. samples | Mean | Min | Max | |
| DM | 6 | 4.9 | 2.8 | 7.1 |
| PH | 6 | 7.1 | 6.5 | 7.4 |
| Hg (ppb/DM) | 6 | 113 | 96 | 156 |
| Cu (ppm/DM) | 6 | 57 | 48 | 71 |
| Zn (ppm/DM) | 6 | 580 | 430 | 840 |
| As (ppm/DM) | 5 | 1.5 | 0.7 | 2.9 |
| Se (ppm/DM) | 5 | 0.1 | 0 | 0.2 |
Table 1b: Cattle slurry composition (adapted from Meeus-Verdinne et al, 1979)
Work will be carried out on both the field site and on small scale, bench-top treatment planes, using batch recycling of dirty wastewater.
It is obvious that the effects of slope variation cannot be determined using the full scale treatment planes in the field. It will be possible, however, to determine the effects of variations in hydraulic and pollutant loading. This can be accomplished both by adjusting the positioning of the cut pipe irrigator and the degree to which the tap is open. Variations in the irrigation to drying time ratio can be investigated; although previous researchers have found little difference in BOD removal rates, the duration of irrigation controls the proportion of anaerobic:aerobic conditions, and this may have implications for nitrogen removal [Walker, 1996]. This may also have implications for phosphorous removal, as phosphorous bound to surface soil particles may be lost from site by washing if flow rates are extremely high or there is continuous surface flow.
Soil respiration can also be investigated on the field site, primarily by the measurement of N 2 O and CO 2 . This will aid in determination of contaminant removal processes, and may allow a tie-in with Sean Tyrrel's work on nitrogen removal from landfill leachate. This aspect also brings attention to the possibility of investigating microbial biomass within the soil, and any correlation between biomass activity and contaminant removal efficiency. As an example of why this may be important, phosphorous uptake efficiency in crops can be improved by enhancement of mycorrhizal symbiosis (Brady & Weil, 1996). Crop uptake may be the primary removal mechanism to prevent soil adsorption sites becoming saturated and phosphorous removal from the dirty water decreasing, as one of the motivations behind this study is to prevent run-off contaminating surface waters with substances such as phosphorous, and removal through erosion to surface waters is the other main removal process from soil (Brady & Weil, 1996).
Compaction of the soil to vary hydraulic conductivity can also be undertaken on selected planes on the field site. This will affect the number of exchange sites available to contaminants in the waste water, surface erosion and possibly plant growth. It may also have an effect on the depth of flow over the plane, and the ability of the irrigation system to ensure sheet flow. It is suggested that the more uniform surface of a higher compaction will promote sheet flow, but may lead to a decrease in pollutant removal efficiency as a result of a reduction in the exchange sites available for removal.
Several researchers have suggested that it is possible to calculate the land requirement for an overland flow slope, notably Smith and Schroeder (1985), adapted in Metcalf & Eddy (1991):
Where:
AS = area required for overland flow slopes (ft2)
Q = average wastewater flow (ft3/h)
VS = net gain or loss in stored volume (ft3/yr)
DS = number of days storage
Ra = design application rate (ft3/h.ft)
Pa = application period (h or d)
Ot = operating cycle time (h or d)
It is proposed that it will be enlightening (or at least entertaining!) to attempt to investigate the rigour of this model as applied to the site.
In the treated effluent, pollutant removal and water losses will obviously be considered. These will primarily include BOD, nitrate- and ammoniacal-N and phosphorous, but it may be wise to investigate metal losses in addition to these. From table 1b, it can be seen that mercury has been found in cattle slurries, mainly as a result of the use of disinfectants. Other metals originate in the cattle feed, and accumulation in the soil will have implications for land use after the life of the system has ended.
At the moment, pre-treatment does not appear to be necessary in all likelihood, as previous workers have found no difficulty in treating effluents of this quality (e.g. Wellsbury, 1994; Walker, 1996), but it is something to keep in mind as part of finding the most efficient use of this system.
The small scale of the treatment planes allows variation in the slope angle which is simply not possible on the large scale. Previous workers have found no real difference in treatment efficacy between slopes of 2-7% (Smith & Schroeder, 1983), and although very little work has been done with grades of greater than 8%, there is no reason to expect a dramatic difference in performance. Successful use of a compound 7% and 12% slope has been reported (Hinrichs, DH et al, 1980), although the risk of erosion and channelling is increased. It is envisaged that the degree of compaction will affect the performance variability caused by changes in slope.
The general water and soil work suggested for the field site should also be carried out on the bench-top planes, including soil respiration. The main difference between the bench-top planes and the field site is the greater degree of control possible, not just over surface slope, but also over hydraulic input (by undertaking experiments under cover it will be possible to avoid dilution by rain and decrease wind-induced evapotranspiration), and soil conditions (factors such as compaction will be much easier to change on the bench-top planes). The bench-top experiments can also be started and under way much more quickly than the field studies.
Both the field site and the bench-top planes will be seeded with Agrostis Stolonifera, which will be harvested and analysed for mineral imbalances. Soil core samples will be taken and the structure determined before the cores are replaced, as stated in the contract, to investigate the effects of the treatment on soil structure.
Factors so far not taken into consideration include reduction of nitrate to dinitrogen gas in the benthic region of the collection tanks, irrigation methods and harvesting frequencies. It is not immediately obvious how these can be investigated thoroughly in the time allotted, but the author is open to suggestions on these and anything else that comes to mind.
Brady, NC & Weil, RC, (1996): "The Nature and Properties of Soils", 11th Ed, Prentice Hall International Inc.
Food and Agriculture Organisation of the United Nations (FAO), (1996): Irrigation and Drainage Paper, 55, "Control of Water Pollution from Agriculture".
Hinrichs, DH et al, (1980): "Assessment of Current Information on Overland Flow Treatment of Municipal Wastewater." Environ. Prot. Agency, EPA-430/9-80-002, MCD 66
Meeus-Verdinne, M, Neirinckx, G, Monseur, X, & de Borger, R, (1979): Real or Potential Risk of Pollution of Soils, Crops, Surface and Groundwater due to Landspreading of Liquid Manure, in "Effluents from Livestock", ed JKR Gasser, Applied Science Publishers, 1980, pp399-408
Metcalf & Eddy, Inc (1991): "Waste Water Engineering: Treatment, Disposal, Reuse" 3rd Ed. McGraw-Hill International.
Smith, RG & Schroeder, ED, (1983): Physical design of overland flow systems. Journal WPCF, 55(3), pp255-260.
Smith, RG & Schroeder, ED, (1985): Field Studies of the overland flow process for the treatment of raw and primary treated municipal wastewater. Journal WPCF, 57(7), pp785-794.
Spallacci, P & Bosci, V (1979): Spreading of Pig and Cattle Slurries on Arable Land: Lysimeter and Field Experiments, in "Effluents from Livestock", ed JKR Gasser, Applied Science Publishers, 1980, pp241-275.
Walker, A, (1996): Soil Microbial Response to Recycled Applications of an Ammonium and BOD rich Landfill Leachate in an Overland Flow Treatment System. MSc Thesis, Silsoe College.
Wellsbury, N, (1994): Effect of Varying Application Regime on Efficiency of Overland Flow Systems for Treatment of Landfill Leachate. MSc Thesis, Silsoe College.