2. Potential Benefits to be Derived from the Utilisation of Sewage Sludges on Agricultural Land
The benefits which may be obtained by the utilisation of sewage sludges on agricultural land include the return of valuable and increasingly scarce, major plant nutrients such as nitrogen and phosphorus to the land. Organic matter, an important constituent of soils, is essential to the maintenance of long term soil structure and fertility. The organic matter in sewage sludge is therefore of value. The Working Party on Sewage Disposal in 1970 (139) encouraged the application of sewage sludge to agricultural land for these reasons. The usefulness of these constituents of sewage sludges was reviewed by the Working Party on the Disposal of Sewage Sludge to Land in 1977 (55) which concluded that the main value of sewage sludge was its nutrient content rather than its organic content. It is possible that in occasional instances, the trace elements present in sewage sludges may be of some value when applied to soils which are deficient in one or more trace elements. Generally however, the trace element content of sewage sludges is of little or no value to agricultural production.
2.1. Soil Structure
2.1.1 Published literature on the effects of sewage sludges on soil structure is scarce. Farmers who were visited during the course of this investigation were found to value sewage sludge mainly for its organic matter content and resultant beneficial effect on soil structure.
This view is held by farmers in Yorkshire, Hertfordshire and Devon. The addition of sewage sludge to soil has been reported to decrease bulk density and to increase the non—capillary pore space (118). Liquid digested sludge can increase the retention of moisture by the soil and so reduce water stress during the growing season (63). Improvements in the aggregation of soil particles resulting from the use of sludge is reported to be of greater benefit than any improvement in the water holding capacity of the soil (118) though at high rates of application a cloddy seedbed can be produced (64).
2.1.2 At one farm in the South—West Water Authority area, a farmer has been applying liquid cold digested sludge to what was very poor permanent pasture land for about 10 years. Previously, there was very little top soil and this was inhibiting the establishment of grass roots. Now, as a result of the application of sludge, a somewhat greater depth of top soil exists and the quality of the sward has been improving annually. The rate of application for this particular site is not known accurately and the distribution of sludge is somewhat uneven due to the steep topography. However, 750 M3 of sludge were applied to 8 hectares over a period of 9 years.
2.1.3 The improvement in soil structure resulting from the application of sewage sludge is most dramatic on poor quality land with thin or non—existent top soil such as exists on sites which consist of coal mining spoil, on worked—out gravel quarries where reclamation is the objective and on uplands, or on poor pasture land.
2.1.4 Recent evidence obtained from the long term Rothamsted Market Garden experiment and reported by Johnston et al., (101) would tend to suggest that the addition of organic matter in sewage sludge to what are normally considered fertile soils, has a beneficial effect on crop yields in the long term.
2.1.5 It is difficult to relate increases in crop yield on other than poor soils, to improvements in soil structure obtained through the addition of organic matter and long term experiments would be required to quantify this potential beneficial effect of the use of sewage sludge. The additional amount of organic matter supplied by sewage sludge when applied to good quality permanent grassland, is likely to be of only minimal value since under grass, soil organic matter tends to accumulate slowly.
2.1.6 Additions of large quantities of organic matter to soil can have several effects on soil physical properties. Although added organic matter will undergo oxidation, substantial increases in soil organic matter levels can be obtained. Where sewage sludge has been disposed of to sewage farms for long periods of time, soil organic matter levels have been increased to 12—15% whereas most soils contain somewhere in the region of 5% or less of organic matter.
The organic matter serves as a granulating agent and can be used to produce a friable and easily cultivated loam soil. The effect is most pronounced on sandy soils of low clay content (15, 64). Improvements of this nature may enable arable and perhaps vegetable crops to be grown in areas where otherwise this might not be possible. Large increases in soil organic matter can cause manganese deficiency particularly at higher soil pH values and instances of this have been observed at a sewage farm and on some West Midland commercial farms. Manganese deficiency is easily corrected however, by application of manganese sulphate solutions.
2.2 Manurial Value
The Working Party on Sewage Disposal (139) reported in 1970 that sewage sludge could supply the equivalent of 4.5% of the country’s total nitrogen and phosphorus fertiliser requirement. Since 1970 fertiliser application rates used in agriculture have tended to increase, particularly nitrogen application rates. Though the nutrient content of sewage sludges varies considerably, if the same composition data utilised for sewage sludge by the Working Party on Sewage Disposal are applied to 1975 fertiliser consumption data (Table 1), then there is reason to believe that somewhat nearer 3.0% of the total fertiliser nitrogen requirements could be supplied by sewage sludge.
At present, only 40% of the sewage sludge produced is utilised as fertiliser, mostly on arable, permanent grass or ley pasture. The area of arable and grassland to which this amount of sewage sludge is applied is estimated to comprise between 1.1% and 1.4% (140,000 — 166,000 ha) of the available 12 million hectares of crop, temporary and permanent grassland in England and Wales (Tables 2,3). This suggests that the approximately 1.5% of the total fertiliser nitrogen requirements supplied by sludge presently applied to agricultural land is being applied at near to optimum rates. It is however, unlikely that sludges presently being disposed to sea or by incineration could be applied at optimum rates, since these sludges presently contain higher concentrations of metals and this would necessitate their application over a proportionately greater area of land to comply with the recommendations of the Working Party on the Disposal of Sewage Sludge to Land (55).
2.2.1 The nutrient content of sludges varies according to the treatment the sludge has received. Work on the comparison of sewage sludges done during the 1940’s at Rothamsted was reported by Bunting (32, Table 4). The sludge used in these experiments originated from the West Middlesex works at Mogden. Sludge produced at any particular sewage treatment works will vary in nutrient content as operational conditions vary. Heated digested sludge from the Wanlip STW is recorded to have contained between 1.6% and 10.7% nitrogen on a dry solids basis (Appendix Al, 1). Similar variations could be expected at other STW.
2.2.2 Comparatively little work has been undertaken on the manurial value of sewage sludges. Early work, using potatoes as a test crop at Rothamsted (32), demonstrated that sewage sludge was little, if anything, more than a source of nitrogen and phosphate, and supplied no potash. This conclusion has been criticized in that potatoes, having a high requirement for potassium, should not have been used as the main test crop (52). However, the percentage increases in yields of carrots and cabbages obtained from the use of sewage sludge were also less than the increases obtained with farmyard manure and straw—sludge composts. The rates of manure application used by Bunting (32) were less than optimum from a manurial point of view. Hence, Bunting was not able to establish whether there was any effect of sludge application on soil physical factors.
Coker (45) indicated that increases in crop yield due to soil improvement by organic manures, were almost entirely restricted to sandy and silty soils which were usually associated with vegetable growing areas.
On a dry matter basis, the nutrient composition of both raw and digested sludges are similar. Compared to farmyard manure and straw—sludge compost, sewage sludge contains little potassium, only about 0.3%. However, in terms of nitrogen and phosphorus sewage sludges compare favourably with other bulky manures, containing as much phosphorus and as much nitrogen as these (32, Table 4).
The digestion process converts approximately half the total nitrogen content of sludge into soluble forms, mainly ammonium nitrogen, which is readily available to crops (45, 52). Treatment of sludge by air drying or by chemical treatment with coagulants, followed by dewatering, generally results in a lowering of the potential nitrogen content of the sludge and represents a loss of nutrients. Only the ammonium nitrogen content of sewage sludge is immediately available for utilisation by a growing crop. Conditioning of sludge with lime and copperas is likely to result in a loss of ammonium nitrogen from sludge, due to the increase in pH.
One of the main advantages of sewage sludge over inorganic fertilisers is the slow rate at which it is mineralised. In this respect, liquid—digested sludges differ from air—dried sludges. During anaerobic digestion, much of the organic nitrogen present in sewage sludge is mineralised to ammonia and hence brought into solution as the ammonium ion. Once added to soil, the sludge ammonium nitrogen can undergo several reactions:
(ii) Adsorption by either soil clay minerals or soil organic matter;
The extent to which ammonia may be volatilized has not been determined, though it will obviously be greater the higher the soil pH is above pH7. The ammonium ion is, however, readily adsorbed by soil particles as well as being absorbed by plants directly. Adsorption of the ammonium ion usually precedes nitrification in soils (115). If sludge or fertiliser is applied at excessive rates of application, there is a remote possibility that the soil exchange complex may become saturated. When this occurs, free ammonium ion will accumulate in the soil solution and may inhibit the activity of Nitrobacter spp., resulting in elevated levels of nitrite in the soil solution. Phytotoxic levels of nitrite in the soil following pig manure applications have been reported (46).
The organic nitrogen in digested sludge presumably becomes available for plant uptake once decomposition and mineralisation takes place, but no significant residual effects have been reported for digested sludge. Dried sewage sludges are only slowly mineralised by comparison. Lunt (118) reported that heat—dried sludges were only slowly nitrified. He further reported that a sludge which had been digested and air—dried, lost its nitrogen at a much lower rate than a sludge which had been digested, treated with ferric chloride and lime, and then vacuum—filtered. Premi and Cornfield (157) were unable to find any evidence of temporary immobilisation of mineral nitrogen due to sludge treatment. The application of organic matter to soil is known to stimulate soil microbial activity. Application of air—dried digested sludge has been found to not significantly affect the soil nitrate level (134) and it was concluded that most of the nitrate produced originated not from the sludge, but from the soil.
Whilst a reduction of ammonium nitrogen content of digested sludge occurs on air—drying, little change in total organic carbon or organic nitrogen occurs. Apart from ammonium—nitrogen, digested sludge is a stabilised material not readily amenable to further biodegradation (143). It is possible that, once the liquid sludge applied to land becomes dry, decomposition is slowed down dramatically. Joffe (94) attributed the retarding effects of drying of soil on mineralisation as being due to enhanced irreversibility of the flocculative and peptizing reactions of the colloidal matter, thus enhancing resistance to microbial attack. The usefulness of sewage sludge as a fertiliser is clearly affected by the rate at which it decomposes when added to soil. The rate of decomposition of digested sewage sludge (as measured by evolution of carbon dioxide) has been found to be largely independent of soil properties other than soil moisture—texture interactions (132). Most of the decomposition was found to occur within one month of the addition of sludge, a maximum of 20% of the added carbon being evolved as carbon dioxide. This may appear to contradict the findings of Molina et al., (143), but implies that some 20% of the sludge organic matter is more readily decomposed than the remaining 80%.
2.3 Soil Fertility
2.3.1. The addition of sewage sludge can benefit overall soil fertility in two ways firstly, by increasing the humus content of the soil with consequent improvements in soil structure and, secondly by increasing the nutrient content of the soil, particularly with regard to nitrogen and phosphorus. The ultimate expression of soil fertility is reflected by crop yield and since yield is affected by the concentration of phytotoxic elements in the soil, it is prudent, given the absence of quantitative yield data, to consider the effect of sewage sludge on the nutrient status of the soil, rather than on crop yield when considering soil fertility per se.
2.3.2 Addition of organic matter to soil usually stimulates soil microbial activity, leading to an increase in biomass. Concomitant with this change, the production of humic acids and polysaccharides is increased (165). Through this mechanism, the soil cation exchange capacity can be increased, the increase in cation exchange capacity of organic matter being dependent largely on soil pH. Significant increases in soil organic matter levels, arising from the application of sludge to a loam textured soil, have been reported at Guelph (191).
In the Woburn market garden experiment, sludge (mostly liquid digested) from the Mogden sewage works, was applied to land at two different rates, single dressings at 75 tonnes1per hectare and double dressings at 150 tonnes per hectare (t ha ). Applications were made once every two years on a continuous basis from 1942 to 1961. Sludge dressings ceased in 1961, when it was apparent that high levels of zinc were accumulating in the soils. During the period of experimentation, 715 and 1180 t sludge /Ha had been applied to the plots, which had received single and double dressings respectively (101). The amounts of organic matter added were respectively 83 and 330 t /Ha (99). While sludge was being applied, soil carbon content increased steadily, from 1.32% to 1.53%, and from 1.70% to 2.12% in the case of single and double dressings, between 1961 and 1960 respectively. In the first nine years of the experiment, increased soil carbon levels accounted for 74% of the increase obtained over an 18—year period. The decrease in rate of accumulation of soil organic matter in the last nine years in which sludge was applied was not explained (99), but would appear to indicate that there was an upper limit beyond which soil organic matter content could not be increased. Once sludge applications had ceased in 1961, soil organic matter levels began to decrease at a rate equivalent to 75% of the average annual gain obtained previously. The percentage increase in soil carbon per tonne of organic matter added was marginally greater for sewage sludge (0.015) than for farmyard manure (0.013). This has been related to the amount of decomposition which had taken place before the manures were added to soil (99). Organic matter added as sewage sludge or compost would be more resistant to further degradation and correspondingly increase soil organic matter levels more so than farmyard manure.
2.3.3 The addition of excessive amounts of ammonium—nitrogen to soil leads to a decrease in soil pH as nitrification takes place. During this process, bases such as calcium, magnesium and potassium are displaced from the cation exchange sites and replaced by hydrogen ions (115). This process makes it more likely that valuable plant nutrients are lost as the soil is leached, transferring the nutrients to depths which are inaccessible to roots or, alternatively, are leached out into drainage waters. Excessive applications of digested sewage sludge can result in the development of anaerobic conditions in the soil, leading to increased mineralisation of the organic nitrogen present in the sludge organic matter. It has been suggested that both the organic and inorganic nitrogen content of sludge should be taken into account in applying sludge to land for this reason (166), despite the fact that anaerobiosis enhances denitrification. Decreases in soil pH, resulting from application of liquid digested sludge to land, have been reported for a sandy clay loam soil (103). Such decreases in soil pH can markedly increase the quantities of exchangeable cations such as zinc, available for plant uptake, irrespective of any trace elements added in the sludge itself. In the Woburn market garden experiment, ground chalk was applied to neutralise any effect that manure dressings might have had on soil pH. It is noteworthy in this connection that farmers no longer receive a lime subsidy.
2.3.4 Large applications of sewage sludge can decrease soil pH. Depletion of soil potassium, magnesium and calcium can accompany a decrease in soil pH. This can be avoided if the soil pH is increased by application of lime, or if sludge application rates are limited in some way. The fact that decreases in soil pH occur concomitant with nitrification justifies the recommendation of the Working Party (55) that sludge should not be applied to soils of much less than pH 6.5.
Digested sludge contains useful bases which ought to be taken into account in assessing its value to crops. No comprehensive assessment of this value would appear to have been made. It is likely that any calcium present in sludge would only be of benefit in cases where application rates did not exceed the nitrogen requirement of the crop, otherwise the addition of calcium in sludge might merely serve to offset the acidification resulting from nitrification except in the case of lime conditioned sludges. Increases in soil acidity resulting from excessive applications of sludge have been reported in the U.S.A. by Chaney (38) and privately by farmers in South—West England.
2.3.5 Little quantitative data exists in the scientific literature on the value of phosphorus present in sewage sludge to crops. Data from the Woburn market garden experiment (99) suggests that much of the phosphorus in sewage sludges is in a comparatively insoluble form, and is not incorporated into the soil by leaching to as great an extent as phosphorus originating from other manures. Nevertheless, work done at Rothamsted on this particular site shows that a linear correlation exists between the total amounts of phosphorus added in sludge and the soil total and sodium bicarbonate soluble (“available”) phosphorus (99).
2.3.6 Whilst soil fertility can be increased by the simple addition of major and minor plant nutrients to the soil, fertility can also be improved by increasing the capacity of a soil to retain nutrients against leaching processes. Continuous applications of sewage sludge might be expected to elevate soil organic matter levels and, as a consequence, the cation exchange capacity of the soil should also increase. Sewage sludge itself has a small cation exchange capacity, but there is little published data which would support the contention that sewage sludge might benefit soil fertility by increasing soil cation exchange capacity. The ability of a soil to retain nutrients such as calcium, magnesium and potassium is largely dependent upon the cation exchange capacity. Recent experiments (64) in the United States showed that soil cation exchange capacity was increased substantially by application of anaerobically digested sewage sludge or dried sewage sludge compost when added at rates approaching 240 t hal. For the silt loan soil used in the experiment soil cation exchange capacity was increased from 5—6 meq 100 /g to 15 meq 100 /g.
2.4 Crop Yields
2.4.1 Most recent experiments in which the effect of sewage sludge on crop yield has been monitored have primarily been concerned with investigating the effect of phytotoxic elements present in sludge, on crop yield; often utilising additions of soluble metal salts to sludge in order to achieve different metal concentrations in the sludge amended soil. This literature is reviewed in another section. Few of these experiments have been of the type which would normally be utilised in assessing the usefulness of an agricultural fertiliser. In many of these investigations moreover the sewage sludges which have been used have been heavily contaminated with metals and the application rates used excessive.
2.4.2 Unfortunately, there is a paucity of information on the effects of varying rates of sludge application on crop yields in the field situation. It has always been assumed that farmers would not use a product which did not enhance crop yields. While this may be true, if water authorities are to charge anything more than a nominal amount for their product, then reasonably accurate estimates of effects on crop yield will be required. Visual assessments of yields cannot be relied upon, as the errors involved are too large. With fluctuations of market prices and yields, it is likely that statistically significant variations of crop yield produced by the utilisation of sludge would not be noticed by a farmer, unless the effect was very large indeed.
The widespread practice whereby farmers ignore the nutrient content of sewage sludge and apply inorganic fertiliser at normal rates of application in addition to sewage sludge adds to the difficulty of trying to assess what influence sludge application has on the productivity of commercial farms.
2.4.3 Increases in yields of cereals of the order of 100% have been obtained on poor quality soils through the use of liquid digested sewage sludge (Cinagro) in the Slough area (42). Substantial benefits of this magnitude are obtainable on poor quality soils from the use of sewage sludge, especially where the soil is (a) sandy or gritty, (b) has a poor capacity for retaining moisture, and (c) is low in organic matter (42).
2.4.4 Bunting (32) found there to be no consistent differences in the usefulness of different types of sewage sludges used in an eight—year period of experimentation. In the comparative study, —l farmyard manure was applied at two different rates: 20 and 40 t ha fresh weight, sewage sludge being applied at 12.5 and 25 t /Ha. In deciding to use these rates of application, a favourable bias towards the value of sewage sludge was built into the experiments by Bunting. Generally, smaller increases in the yields of potatoes, red beet, carrots and cabbage were obtained when sewage sludge was used in place of farmyard manure or straw/sludge compost (Table 5). Table 6 shows some further results for the years 1944—1960 for the Woburn market garden experiment. The results shown are for the lowest of the two rates of application of manures and sludge. Sewage sludge would appear to be particularly beneficial in increasing cabbage, beet and leek yields. Increases in yields of barley, potatoes and sugar beet arising from application of bulky organic manures (of which sewage sludge was one) have also been reported for the Woburn market garden site (100). It has been reported that the yields in this case were dependent to a very large extent upon the amount of bicarbonate—soluble P in the soil (Figure 1). Increases in crop yields arising from the use of sewage sludge as a fertiliser cannot, in most cases, be explained solely by a single factor. Johnston et al., (100), commented that the increased amount of organic matter present in the soil was probably responsible for most of the increases in yield obtained in this experiment. These results would certainly seem to suggest that the value of increased organic matter levels in light texture soil has been underestimated. One reason put forward to explain the increased yields was that the extra organic matter may have made it easier for roots to penetrate between soil particles and so explore a greater mass of soil for nutrients. Table 7 (52) shows the amounts of organic matter and nutrients added to soil by application of sewage sludge suited to agricultural conditions.
2.4.5 The high ammonium—nitrogen content of liquid anaerobically digested sludge makes this type of sludge very suitable for utilisation on grassland. The usefulness of liquid digested sludge as a fertiliser for mixed grass—clover pastures and on intensive ryegrass production has been investigated by Coker (43, 44) utilising sewage sludge from the Rye Meads STW. Percentage recovery of applied nitrogen by mixed grass—clover sward from sewage sludge compared favourably with that obtained from equivalent rates of inorganic fertiliser application, when low nitrogen application rates were used (71—76 Kg N /Ha) but slightly less favourably at high rates of application of nitrogen (120 Kg N /Ha). The increase in dry matter production at low rates of application of nitrogen was greatest when supplied by sewage sludge (Table 8). At low rates of sludge application clover was less suppressed than at equivalent inorganic fertiliser application rates (43) but at least one farmer has reported deleterious effects of sewage on the growth of clover.
Application of excessive quantities of nitrogen (greater than 100 Kg hal) to cereal crops may cause lodging or increase susceptibility to lodging in bad weather. At rates of application of sewage sludge below this level, yields of barley straw and grain are comparable to those obtained with inorganic fertiliser at equivalent rates of application though recovery of applied nitrogen is less from sewage sludge (43).