4. Review of Current Practices and Past Experience of the Disposal of Sewage Sludge to Agricultural Land in the United Kingdom
Since the re-organisation of the water industry in 1974, the disposal of sewage sludge has been more systematically organised. Prior to that date, detailed records of individual sludge disposal operations were not usually kept. Proper historical data exists for only a few sites in the United Kingdom.
The sites for which historical data is available are those which in the past have been used for sewage disposal and more recently for sludge disposal. They have received large quantities of both at unknown rates of application, but sufficiently high to considerably elevate soil organic matter levels,-and indeed the soil on some of these sites has largely been formed from sewage and sewage sludge.
The high organic matter levels to be found in these soils distinguish them from most normal agricultural soils. High soil metal contents as a result of uncontrolled sludge applications further distinguish these soils from typical agricultural soils. These two criteria effectively serve to ensure that any conclusions made from studies of these sites cannot be assumed to hold in a truly agricultural situation, where soil organic matter and soil metal contents are generally much lower.
4.1 Application of Sludge to Land
4.1.1 Some sewage treatment works produce sludge in a dry powder or granular form. This is because the plants required for processing were inherited by the Regional Water Authorities when re-organisation took place in 1974. Two such works exist in Yorkshire: Dewsbury and Halifax (appendix AS; 2a, 2b). The material produced at Halifax is a heat treated and thermally dried primary/secondary sludge, with a moisture content of about 15%. The sludge contains approximately 3% nitrogen, 5% phosphorus and 1% potassium on a ù59ù
dry weight basis. Sludge produced at the Dewsbury works has a similar composition, but is of a granular nature and is therefore more suited to horticultural uses. It is for this reason and the need to recover processing costs that it is marketed. The Thames Water Authority is currently planning to market a dried processed sludge in the South East (Farmer’s Weekly, 2nd September, 1977). It is unlikely that new plants will be built to produce dried sludges because of the high capital costs incurred. Nevertheless, market outlets appear to be readily available for the material currently produced.
4.1.2 Unless adequately dried, sewage sludge cake presents considerable difficulties in achieving incorporation with the soil when compared to liquid sludges, necessitating increased handling and soil cultivation. The Working Party (55) drew attention to the fact that wet sticky sludges would also be the most difficult to handle and distribute.
4.1.3 There is little difficulty in transporting liquid sludges which are usually more acceptable forms for agricultural use, enabling spreading by conventionally drawn trailers, 4ùwheel drive tankers and by conventional agricultural irrigation equipment. The latter is useful in wet weather where soil structural damage could be expected to result from the use of tankers.
Liquid sludges are mostly transported to agricultural land in 4,500 gallon (20 in3) tankers. In dry weather, these tankers may be used for the application of sludge. In wet weather or when the soil is unsuitable for the use of such heavy vehicles, smaller 800-1,000 gallon (3.5 - 4.5 M3) slave tankers may be used for application.
If irrigation equipment is to be used to apply sludge, it is important to ensure that sludge is well screened to prevent any blockages of nozzles by pieces of rags or plastic material. Such blockages would cause serious difficulties in the field operation.
4.1.4 On sites which have a history of sewage and sewage sludge application, for example Northampton, Great Billing, Dudley, and Haydon Knoll, the application techniques used in the past have been empirical. Flood irrigation, either straight from a discharge pipe or by ensuring distribution by means of ploughed channels has principally been employed. Usually the resulting distribution has been rather uneven and this has resulted in the development of ‘pockets’ of soil with elevated concentrations of organic matter or metals.
4.1.5 No instances of surface water pollution as a result of the application of liquid sludges leading to run-off into rivers or streams have been reported. It is considered unlikely that such pollution would occur, given that sludge is applied by conventional agricultural means and not spread on steeply sloping ground or on ground a4jacent to rivers. However some cases of groundwater pollution have been reported as a result of sewage sludge application.
4.1.6 Application of liquid sludge by raingun could result in water pollution if used near streams or rivers or in strong wind conditions. Careful attention on the part of operators should minimise this potential hazard. Likewise, when used close to residential areas it is important to pay close attention to wind strength and direction so as to avoid public nuisance.
4.1.7 Whilst contamination of groundwater can be avoided or minimised when suitable precautions are taken, care should be taken to ensure that sludge is not applied close to any unlicensed boreholes for which information may not readily exist.
4.1.8 The Working Party (55) advised that ploughing to a depth of about 200 mm following the spreading of sludge on land for growing crops would assist incorporation into the soil. Close to residential areas, such incorporation would also serve to minimise any potential odour nuisance.
4.2 Monitoring of Sludges
4.2.1 Sewage sludges are analysed at intervals for those elements for which adequate standard methods of analysis are available: zinc, copper, nickel, chromium, cadmium, and lead. Of the additional elements mentioned in the Working Party Report (55), analyses for mercury, molybdenum, arsenic, selenium and boron are currently time consuming, technically difficult and are only occasionally performed. Boron is unlikely to cause toxicity to plants when applied in sewage sludge at rates of application consistent with good agricultural practice. Mercury, molybdenum, arsenic and selenium all have very specific industrial uses and generally occur at very low levels, in sewage sludges. Frequent monitoring for the last group of elements is thus unlikely to be necessary, although checks on sludge composition ought to be performed at intervals.
4.2.2 The methods used for metal analyses and particularly those employing acid digestion vary considerably. Until such time as standard methods become universally employed within the water industry the actual methods used in analyses should always be stated in reports and records to enable adequate comparison of results.
4.2.3 From the agricultural viewpoint, sludge nutrient analyses should be made available at intervals to farmers. This is to some extent already done and usually includes analyses for total nitrogen, phosphorus (usually expressed as P205) and potassium (as K20). The total nitrogen content is of only limited use to a farmer. The total soluble nitrogen content would be of much more use in calculating appropriate rates of application to agricultural land. Similarly the farmer should be supplied with details of neutralising value (expressed as CaCO3) of the sludge and its magnesium concentration. The organic matter content of sludge, expressed as volatile solids should also be made available to the farmer. ù61ù
4.3 Monitoring of Soils and Crops
4.3.1 Analysis of soils prior to sludge application was not undertaken for sites with a history of sludge amendment. This makes it difficult to relate any observed effects to the application of sludge.
4.3.2 With the exception of the Whitehall and Haydon Knoll farms of the Severn Trent Water Authority at Stoke Bardolph (Table 12) no detailed records of crop yields have been made available for sites with a history of sludge application.
4.3.3 In the U.K., only limited information dealing with the analyses of the elemental composition of crops grown on sludge amended soils exists. Detailed information was found for only four sites which have a history of sewage and sewage sludge application. These are the Northampton and Peterborough farms of the Anglian Water Authority, the area round Perry Oaks in the Thames Water Authority area and the Stoke Bardolph farms of the Severn Trent Water Authority (Tables 9, 10 and 12 respectively). As mentioned above, corresponding yield data exists only for the latter site.
4.3.4 The data which is available for the elemental composition of crops grown on these sites indicates that metals are not accumulated in the edible parts of plants to the extent which studies using soluble metal salts would suggest.
4.3.5 Water Authority records indicate that sewage sludge is often applied to soils with a pH below 6.5, whereas the Working Party repeated the Ministry of Agriculture’s recommendation that soil pH should be maintained at pH 6.5 for arable crops and at 6.0 for grassland. The Working Party itself recommended that soils known to be contaminated should be maintained at a pH in excess of 7.0 if possible (55).
4.3.6 Where sewage sludge disposal is being contemplated on green field sites the general policy of some Regional Water Authorities is for the analysis of soils prior to sludge application and at intervals following the application of sewage sludge of known composition.
4.3.7 Where soil analyses are being performed some water authorities also undertake analyses for ‘available’ metals in accordance with the methods outlined in A.D.A.S.’ Advisory Bulletin 10(41).
4.4 Practical Implication of Guidelines for Sewage Sludge Disposal to Agricultural Land
4.4.1. The major practical implication which the existence of guidelines creates for the disposal of sewage sludge to land is that of limiting permissible application rates and perhaps, requiring that sludge be distributed over a larger area of land. Secondly, the existence of guidelines creates the need for extensive monitoring.
4.4.2 The successful and safe utilisation of sewage sludge on agricultural land depends on the compliance with suitable guidelines and is limited by the adequacy or otherwise of laboratory facilities required for the satisfactory monitoring of the operation. A major impression gained in the course of this study is that such facilities are often inadequate either as a result of a lack of material resources or of trained personnel.
4.4.3 For the purpose of calculating permissible or acceptable additions of sludge to land, the Working Party (55) advised that it could be assumed that 85% of the nitrogen in liquid digested sludges was available to crops within the growing season. The evidence for this assumption originates from only one series of experiments on a limited number of crops (43, 44). It is possible that this figure is an overestimate of what is readily available to crops. It would be better to rely on the total soluble nitrogen content of liquid sludges as a guide to acceptable rates of application and not to make assumptions, as sludges will inevitably be mineralised at different rates on different soils and in different climatic conditions.
4.4.4 The Working Party (55) recommended that land which has received untreated sludge should not be grazed for a period of 6 months following application, unless adequate monitoring for pathogens shows that a shorter period is acceptable. This is likely to cause severe difficulties in the disposal of sludge to land in certain parts of the United Kingdom. In Wales, the South-west and other areas where permanent pasture comprises a large percentage of the land ~available for sludge disposal operations, the difficulties will be greatest.
4.5 Farmers Views and Comments
In the course of the visits undertaken as part of this survey, the personal views of a limited number of private farmers who have used sewage sludge on their land have been gathered. While these opinions are recorded here, it must be noted that they neither reflect facts supportable by scientific data nor represent a significant random sample of farmers.
4.5.1 It has been stated by various farmers that they or their colleagues value sewage sludge for the following reasons, not necessarily in order of priority:
i) Organic matter ii) Nutrients ,especially phosphorus iii) Lime content iv) Irrigation
4.5.2 Several farmers have stated that they would not accept sewage sludge if any payment were requested.
4.5.3 It has also been pointed out that sewage sludge could be more precisely incorporated into farming practice if more information on its agricultural value was available. Moreover this information should be available for each type and delivery of sludge. ù63ù
4.5.4 Several farmers have indicated that despite the use of sewage sludge they maintain normal application rates of artificial fertilisers.
4.5.5 Some farmers are under the misapprehension that anaerobic digestion kills all pathogens.