LAND DISPOSAL AND AGRICULTURAL REUSE OF SEWAGE SLUDGE WITHIN THE FRAMEWORK OF THE CURRENT SOUTH AFRICAN GUIDELINES

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1 Presented at WISA 2000 Biennial Conference, Sun City, South Africa, 28 May 1 June 2000 LAND DISPOSAL AND AGRICULTURAL REUSE OF SEWAGE SLUDGE WITHIN THE FRAMEWORK OF THE CURRENT SOUTH AFRICAN GUIDELINES Heidi G. Snyman 1, W.V. Alexander 2 and C.J. Marx 3 1 ERWAT Chair in Wastewater Management, Room 1.34, Building 2 South Campus, Water Utilisation Section, Department of Chemical Engineering, University of Pretoria, Pretoria, Tel: +27(12) , Fax: +27(12) Steward Scott (Sludge Consult), PO Box , Sandton, Tel: +27(11) , Fax: +27(11) Africon (Sludge Consult) PO Box 905, Pretoria, Tel: +27(12) , Fax: +27(12) ABSTRACT Disposal and reuse in the agricultural sector had to comply with the Guide: Permissible Utilisation and Disposal of Sewage Sludge (1991). These sludge guidelines were revised in 1997 with considerable amendments to the heavy metal loading and usage restrictions. Analysts continued using the aqua regia extraction method, which made all plants seem to be non-compliant, and could not generate a Class D sludge. This caused the industry to seek alternative non-beneficial disposal methods. This paper aimed to clarify the analytical requirements and the available options for the beneficial use of sewage sludge. Metal extraction using the Toxicity Characteristic Leaching Procedure (TCLP) method was compared to the aqua regia extraction method and the extent of compliance of plants were extrapolated from this data. A misperception that sludge can only be used beneficially when it complies with a Class D was clarified illustrating the available disposal and utilisation routes in accordance to the legal requirements. The link between the sludge guidelines and the minimum requirements for handling, classification and disposal of hazardous waste was also documented. KEYWORDS Agriculture; beneficial use; metal content; sewage sludge. INTRODUCTION The ultimate disposal of wastewater sludge (biosolids) continues to be one of the most difficult and expensive problems in the field of wastewater engineering (Tchobanoglous and Burton, 1991). Application of sewage sludge to agricultural land is seen as an economical beneficial re-use method. The major benefits of sludge application are; increased supply of major plant nutrients; provision of some of the essential micronutrients (Zn, Cu, Mo, and Mn); and improvement in the soil physical properties, i.e. better soil structure, increased soil holding water capacity, and improved soil water transmission characteristics (Korentajer, 1991). A gradual decrease in the organic matter content of cultivated soils in the world, as a result of factors such as excessive use of inorganic fertilizers, is a worldwide phenomenon. In warm climates, such as South Africa, this process is accelerated due to rapid microbial

2 decomposition of the soil organic matter. The decrease in soil organic matter content is a problem of major concern since it may lead to a deterioration of the soil physical structure and accelerated erosion (Korentajer, 1991). Sacrificial land disposal and reuse in the agricultural sector are still popular methods for disposing of excess sewage sludge in South Africa. These practices had to comply with the Guide: Sewage Sludge, Utilisation and Disposal (1991). These guidelines were revised in 1997 with considerable amendments to the heavy metal loading and usage restrictions. The guidelines deal exclusively with application of sewage sludge to land and amongst other criteria set permissible limits for concentration of heavy metals in sludge suitable for unrestricted (limited to 8 ton/ha/year) agricultural land use. It contained guidelines for sewage sludge disposal, which have subsequently been sited by the Department of Water Affairs and Forestry as the only acceptable criteria for sewage sludge disposal for licence applications. The metal limits in the sludge guidelines has up to now been interpreted at total metal content. However, this paper outlines the origin of some of the limits in the guideline and attempts to clarify the method of sludge analysis. The paper also addresses preliminary results for the suggested analytical methods compared to the traditional method for sludge analysis. SOUTH AFRICAN SLUDGE GUIDELINES The Permissible Utilisation and Disposal of Sewage Sludge, Edition 1 (1997) was aimed to assist organisations involved in sewage treatment to promote safe handling, disposal and utilisation of sewage sludge. Sewage sludges are classified in four categories according to the potential to cause odour nuisances, fly breeding and transmit pathogenic organisms to man and his environment. Sludge of Class A to C can be utilised as a soil amendment under certain restrictions and the sludge producer remains responsible for the safe handling and disposal/use of the sludge. Type D sludge can be used without restrictions up to 8 ton/ha. Some of the maximum permissible metal contents for sludge aimed for unrestricted use (type D) were amended considerably (Table 1). Table 1: Comparison between the metal content of sludge aimed for unrestricted use (Type D) in the 1991 and 1997 sludge guidelines (Guide: Permissible utilisation and disposal of sewage sludge, 1991; Permissible utilisation and disposal of sewage sludge, 1997). Metal 1991 Limit (mg/kg dry sludge) 1997 Limit (mg/kg dry sludge) Cd Co Cr Cu Hg Mo Ni Pb Zn As Se B F

3 For example, the maximum permissible copper content was reduced from 750 mg/kg to 50.5 mg/kg. The new metal limits were derived based on a risk factor calculation to minimise the risk to the aquatic environment (Department of Water Affairs and Forestry, 1998). Typically, analytical laboratories were not informed that the method of analyses have changed, and hence continued to use a strong acid extraction to determine the metal content of sludge. This caused that not one wastewater treatment plant in South Africa complied with the 1997 Guidelines (Snyman et al., 1999). An official document of comments and concerns was handed to the Department of Water Affairs and Forestry and a project was launched to investigate the origin of the amended limits and analytical methods. EFFECT OF THE AMENDED METAL CONTENT ON SLUDGE COMPLIANCE The effect of the amended metal content on the compliance and therefore permissible use is illustrated in the following section. Data collected by Smith and Vasiloudis (1989) from 77 South African wastewater treatment works were analysed to illustrate the effect of the new sludge metal limits for a type D sludge. The general sludge characteristics are listed and the discussion concentrated on the revised metals, that is cadmium, copper, lead and zinc. Sludge characteristics The general inorganic characteristics of sludge generated at South African wastewater treatment works are listed in Table 2. The variation of constituents in the sludge is evident in the standard deviation values given in Table 2. The wastewater treatment plants used in the study were selected to include urban and rural cities as well as works receiving domestic and/or industrial wastewater. Table 2: Sludge characteristics derived from data of 77 South African wastewater treatment plants (Smith and Vasiloudis, 1989). Parameter Concentration Standard deviation Total Kjeldahl Nitrogen Total Phosphorus Potassium Cadmium Chromium Copper Lead Nickel Zinc Mercury 5 4 Arsenic 7 5 Selenium 3 12 Molybdenum 7 4 Boron Fluoride Toxic metals, in particular Cd, Cu, Mo, Ni, Pb and Cr are normally present in sewage sludge. Heavy metals may be transmitted in the food chain and because of the high toxicity present a threat to crop production and animal and human health. In particular, Cd, because of its

4 relatively high mobility in the soil environment, is considered to be the element most likely to limit application of sludge on land (Korentajer, 1991). Table 3 indicates that Cd occurs in relatively low concentrations in South African sludges compared to the metals such as Cu, Ni, Pd and Cr. Cadmium The permissible cadmium concentration for a type D sludge (Table 1) was revised from 20 mg/kg to 15.5 mg/kg. The average cadmium concentration of sludges from 77 wastewater treatment plants throughout South Africa derived from data collected by Smith and Vasiloudis (1989) is depicted in Table 2. These cadmium values were determined using atomic absorption and extracted with aqua regia solution. The determined value therefore gives an indication of the acid soluble fraction of the metal in the sample. However, the revised value of 15.7 mg/kg for cadmium is the acceptable risk level, which is derived as 0.1LC 50 (Department of Water Affairs and Forestry, 1998). Ten percent (0.1) of the LC 50 represents the concentration at which the risk to the population would be one in The value of 15.7 mg/kg is therefore the acceptable risk level to an aquatic environment or the maximum concentration that could leach out into the environment. To measure the amount of a substance, which could potentially, leach out, the Toxicity Characteristic Leaching Procedure (TCLP)(1990) is used. The TCLP method was developed to measure a waste s leachability and hence risk it poses to the groundwater. This method has historically not been used by the wastewater industry to assess the amount of sludge to be safely used in agricultural practices. Most wastewater laboratories have used the aqua regia extraction, which gives an indication of the acid soluble fraction. To compare the methods, 4 samples, a waste activated- (WAS) and anaerobically digestedsludge (ADS) sample from two different wastewater treatment plants were collected. These were extracted using the TCLP no 2 solution (ph 2.88) (Department of Water Affairs and Forestry, 1998) and aqua regia solution (ph < 0) and analysed on a auto analyser (Table 3). Table3: Cadmium concentrations of sludge samples extracted with TCLP and aqua regia solution. Sludge Aqua Regia Extraction TCLP Extraction TCLP/aqua regia WAS % WAS % ADS % ADS % Table 3 indicates a considerable variation in the ratio between the two extraction methods. On average the amount of Cd captured by the TCLP extraction compared to the aqua regia extraction is 7.1% ± 9.6. Two of the sludges would not have complied with the 1991 guidelines, while all the sludges comply to the 1997 guidelines if the TCLP extraction method is used. If the data from Smith and Vasiloudis (1989) is used and these values are extrapolated for possible TCLP extractable concentrations, all 77 plants would comply to the 1997 guidelines, while 83% of the samples complied with the 1991 guidelines. This indicates a significant lowering of the guideline limit in terms of permissible cadmium concentration in sludge.

5 Theoretically, a sludge sample could contain cadmium values up to 200 mg/kg using the traditional aqua regia extraction and still comply with the guidelines. This value is an order of magnitude higher than limits set by the rest of the world (Matthews, 1997). Copper The permissible copper concentration for a type D sludge (Table 1) was revised from 750 mg/kg to 50.5 mg/kg. The average copper concentration (aqua regia extracted) of sludges from 77 wastewater treatment plants across South Africa derived from data collected by Smith and Vasiloudis (1989) is depicted in Table 2. Table 4: Copper concentrations of sludge samples extracted with TCLP and aqua regia solution. Sludge Aqua Regia Extraction TCLP Extraction TCLP/aqua regia WAS % WAS % ADS % ADS % Table 4 indicates the amount of Cu captured by the TCLP extraction compared to the aqua regia extraction (7.6% ± 7.2). The sludge samples in Table 5 complied with both the 1991 guidelines and 1997 guidelines. If the data from Smith and Vasiloudis (1989) is used and these values are extrapolated for possible TCLP extractable concentrations, 83% of the plants would comply with the 1997 guidelines. The same amount of samples complied with the 1991 guidelines. In terms of the permissible Cu concentration the effect of the analytical method change did not influence the compliance of the plants. Lead The permissible lead concentration for a type D sludge (Table 1) was revised from 400 mg/kg to 50.5 mg/kg. The average lead concentration (aqua regia extracted) of sludges from 77 wastewater treatment plants throughout South Africa derived from data collected by Smith and Vasiloudis (1989) is depicted in Table 2. Table 5: Lead concentrations of sludge samples extracted with TCLP and aqua regia solution. Sludge Aqua Regia Extraction TCLP Extraction TCLP/aqua regia WAS % WAS % ADS % ADS %

6 Table 5 indicates the amount of Pb captured by the TCLP extraction compared to the aqua regia extraction (1.1% ± 1.52). The sludge samples in Table 5 complied with both the 1991 guidelines and 1997 guidelines. If the data from Smith and Vasiloudis (1989) is used and these values are extrapolated for possible TCLP extractable concentrations, 99% of the plants complied with the 1997 guidelines. The same amount of samples complied with the 1991 guidelines. In terms of the permissible Cu concentration the effect of the change in analytical method did not influence the compliance of the plants. However, if the TCLP method were chosen as the preferred analytical method to analyse for Pb, a plant producing a sludge with an aqua regia extractable concentration of 4500 mg/kg will still comply with the TCLP extractable limit. Zinc The permissible zinc concentration for a type D sludge (Table 1) was revised from 2750 mg/kg to mg/kg. The average zinc concentration (aqua regia extracted) of sludges from 77 wastewater treatment plants throughout South Africa derived from data collected by Smith and Vasiloudis (1989) is depicted in Table 2. Table 6: Lead concentrations of sludge samples extracted with TCLP and aqua regia solution. Sludge Aqua Regia Extraction TCLP Extraction TCLP/aqua regia WAS % WAS % ADS % ADS % Table 6 indicates the amount of Zn captured by the TCLP extraction compared to the aqua regia extraction (8.3% ± 2.9). The sludge samples in Table 6 complied with both the 1991 guidelines and 1997 guidelines. If the data from Smith and Vasiloudis (1989) is used and these values are extrapolated for possible TCLP extractable concentrations, 82% of the plants complied to the 1997 guidelines while 92% of the plants complied with the 1991 guidelines, indicating a lowering of the limit. ULTIMATE DISPOSAL AND UTILISATION OF SEWAGE SLUDGE It is generally believed that if sludge does not comply with the specified metals of Class D, it cannot be used beneficially. It is in fact possible to use sludge of a lower quality than Class D beneficially if it is handled responsibly. Diagram 1 illustrates a decision-making flowchart, which could assist the industry with sludge handling and different possible applications thereof. Diagram 1 illustrates where the Minimum Requirements (Department of Water affairs and Forestry, 1998) and hence the TCLP method of characterisation plays a role. This method of characterisation plays a very important role when sludge is disposed in landfill or through sacrificial land disposal. CONCLUSIONS

7 South Africa is considered an arid country with an average rainfall of 497 mm/yr. The country is therefore forced to reclaim as much water as possible, which is done by using biological wastewater treatment in most cases. The sludge generated can be used to enhance the soil condition and water holding capacity, therefore preventing further deterioration of the soil structure through erosion. However, sewage sludge contains metals which, have to be measured, to ensure that the beneficial use is not irresponsible. The environmental effects of sewage sludge applied to agricultural land has been well documented including the stabilising effect soil has on sludge (Snyman et al., 1998) and the effect of accelerated sludge application on land (Henning et al., 1999). Lötter and Pitman (1997) have researched the long-term effects of sludge application to land. When the guidelines of 1991 were revised in 1997, analysts continued to use the aqua regia extraction method which made all plants seem to be non-compliant and could not generate a Class D sludge (Snyman et al., 1999). This caused the industry to seek alternative non-beneficial disposal methods. This paper aimed to clarify the analytical requirements and the available options for the beneficial use of sewage sludge. Metal extraction using the TCLP method was compared to the aqua regia extraction method and the extent of compliance was extrapolated from this data. A misperception that sludge can only be used beneficially when it complies with a Class D was clarified illustrating the available disposal and utilisation routes in accordance to the legal requirements. AGKNOWLEDGEMENTS The authors wishes to acknowledge Department of Water Affairs and Forestry and the University of Cape Town, Department of Civil Engineering (G Ekema) for their valuable contributions and discussions.

8 Sludge Producer Quantity Quality Processes Determine mineral quality of sludge and cautionary remarks Implement better industrial effluent control Can the sludge metal content be improved by further industrial control? Do you conform to relevant legislation? Will plant be extended? Continue with current disposal method Apply for permit or licence Metal content less than D Classify sludge in terms of metals < D or > D Metal content more than D Can you mitigate to class D? Do you want to dispose of or use sludge? Dispose On site or off site On site Minimum requirements Treatment : composting, pelletisation, etc. Will the sludge be used in agriculture? Will sludge be used in industry? Agricultural Permits + EIA + inputs from agriculture health etc. Use Agricultural or industrial Industrial Off site Section 20 permit to remove Minimum requirements Site surveys, EIA's, permits, etc. Treatment/ Mitigating engineering Is it a class D sludge? Register as fertiliser Treatment/ Mitigating engineering Landfill on hazardous waste Delisting Land disposal Lagoons Other (Incineration) Product market Edible crops n-edible crops Other EIA's permits Treatment/ Mitigating engineering Permits EIA Co-disposal Land rehabilitation Other Treatment and/or mitigating engineering Bricks Cement Gasification Register as fertiliser Other Market product Diagram 1: The sludge disposal and utilisation decision flow diagram

9 REFERENCES Department of Water Affairs and Forestry. (1998). Waste Management Series. Minimum Requirements for the Handling, Classification and Disposal of Hazardous Waste. 2 nd edn., Pretoria, South Africa. Guide: Permissible Utilisation and Disposal of Sewage Sludge. (1991) Department of National Health and Population Development, A11/2/5/4 (2 nd Draft), Pretoria, South Africa. Henning B., Snyman H.G. and Aveling T.A.S. (1999). The cultivation of maize (Zea mays L.) on high sewage sludge dosages at field scale. Proceedings of Specialised Conference on Disposal and Utilisation of Sewage Sludge: Treatment Methods and Application Modalities. Athens, Greece. Korentajer L. (1991). A review of the agricultural use of sewage sludge: benefits and potential hazards. Water SA, 17(3), Lötter L.H. and Pitman A.R. (1997). Aspects of sewage sludge handling and disposal. Water Research Commission, Report 316/1/97, Pretoria, South Africa. Matthews, P (1997). A global atlas of wastewater sludge and biosolids use and disposal. IAWQ Scientific and Technical Report. 4 Permissible Utilisation and Disposal of Sewage Sludge, 1 st edn. (1997). Water Research Commission, Report TT 85/97, Pretoria, South Africa. Smith R. and Vasiloudis H. (1989). Inorganic Chemical Characterization of South African Municipal Sewage Sludges, Report 180/1/89, Water Research Commission, Pretoria, South Africa. Snyman H.G., De Jong, J.M. and Aveling, T.A.S. (1998). The stabilization of sewage sludge applied to agricultural land and the effects of maize seedlings. Wat. Sci. Tech., 38(2), Snyman H.G., Terblanche, J.S. and Van der Westhuizen J.L.J. (1999). Management of land disposal and agricultural reuse of sewage sludge within the framework of the current South African guidelines. Proceedings of IAWQ Specialised conference on Disposal and Utilisation of Sewage Sludge: Treatment Methods and Application Modalities. Athens, Greece. Tchobanoglous G. and Burton F.L. (1991). Design of facilities for the treatment and disposal of sludge. In: Wastewater Engineering. Treatment, Disposal and Reuse. 3rd edn, Metcalf and Eddy, Inc. McGraw-Hill, Inc., New York, pp Toxicity Characteristic Leaching Procedure. (1990) Federal Register, 51(114),