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Ecological and Economical Relevance of Sludge Treatment and Disposal Options H. Kroiss and M. Zessner Institute for Water Quality and Waste Management at Vienna University of Technology, Karlsplatz 13, A 1040 WIEN, Austria (E-mail:
[email protected]) Abstract: Solutions for sludge treatment and disposal produced at waste water treatment plants have to be reliable at any time and therefore need a legally, organisationally and technically sound background. The legal background normally is created at a national or even supranational (EU) level. The technical and organisational solutions can be manifold depending on the specific local situation at a treatment plant. This paper deals with the development of indicators for sludge treatment and disposal which enable the decision makers at a national level to assess sludge disposal options in regard to economic and ecologic relevance. Special emphasis is given to the use of sludge in agriculture as it follows the ideas of recycling of valuable nutrients from waste water to agriculture. This investigation shows the different relevance of sludge disposal for the treatment plant operators and for agriculture in terms of economy and reliability. For the treatment plants sludge disposal represents a very important cost factor much less for the “consumers” of waste water services. For agriculture sludge is of low economic relevance while for a farmer using sludge as P-fertiliser it can be high. The most relevant ecologic aspect of sludge disposal is with the P-content of sludge which has a high relevance in regional material cycle while it is low for Nitrogen, the value of organic matter in the sludge has only local/regional relevance. The method developed can be applied for other countries. The results for Austria (EU member state) are calculated and discussed in detail. Keywords: decision support, sludge treatment and disposal, economic and ecologic indicators
INTRODUCTION Sewage sludge disposal has always been closely linked to an asymmetric discussion between waste water technology and agriculture. Waste water treatment experts and managers try to convince agriculture that sewage sludge is rich in nutrients and other valuable compounds and are praising the idea of nutrient recycling by agricultural sludge utilization. Farmers and especially the political representatives of farmer unions do not deny the arguments of the waste water experts but claim that there is no need to use sludge as it does not contribute to increase their income even they admit that some farmers could make profit from it. The primary interest of treatment plant operators is to have a reliable sludge disposal route at any time and at reasonable cost. They have very little influence on sludge production as it is primarily the consequence of fulfilling a legal requirement (KROISS 2004). Farmers are interested in stabilizing and increasing their income from crop and animal production, to achieve a good price for their products and to maintain soil fertility over long periods of time. The main argument to ban agricultural use of sewage sludge on land is its contamination with potentially hazardous compounds. The sources of these compounds are manifold and reflect our life-stile. Micro-pollutants reach the waste water via our nutrition, our use of pharmaceuticals, personal care products and household chemicals and finally via diffused sources as building materials (heavy metals), road erosion and car traffic. Air pollution, too, contributes to the trace pollution of waste water and hence sewage sludge. Air pollution reaches agricultural land also via dry or wet deposition. Even “best agricultural practice” does not prohibit soil “contamination” by the use of chemicals (pesticides, herbicides, etc.) market fertilizers, fodder, manure (nutrients, pharmaceuticals, disinfectants, heavy metals) and organic material (compost, peat). Also private waste management companies competing on the market take care of sludge disposal. At an increasing number of treatment plants sludge disposal is contracted with these companies. They have developed markets for products (compost, “soil”, landscaping materials), operate waste incineration plant or co-operate with cement or building material producers. With these products sludge compounds reach natural soils, landfill disposal or constructions. Therefore stringent quality control and risk management is necessary in order to protect consumers and the environment. These new sludge-stakeholders have interest in good 47
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relations to treatment plant operators, framers and other users of their products in order to meet their commercial goals. The discussion on an adequate quality of sewage sludge for agricultural application has led to tremendous improvement of sludge quality by strict source control of heavy metals and industrial and trade effluents and by the ban of certain materials as cadmium as corrosion inhibitors. E.g. the concentrations of the most toxic heavy metals as mercury, cadmium and lead have dramatically decreased during the last 3 decades and today are close to or even below soil standard concentrations. Wherever strict source control of micro-pollutants is applied their concentrations in the sludge are quite similar and low. The risk management strategy behind the 1986 Sludge Directive was extensive research on cause relationship between sludge application and the response of soil, plants and crop quality with a mid term perspective allowing to interfere if new findings suggest changes. An application of 1 - 2.5 t DS/ha/a can cover the phosphorus requirements of agricultural production depending on P removal requirements. The dilution of the sludge with the soil material is in the order of 1:10 000. As a consequence changes in soil composition caused by the use of sludge can only be proved by analytical methods after a decade or more of sludge application. Risk assessment, therefore, cannot be based on monitoring data alone and there will be always an unknown risk left. In order to enhance agricultural sludge application a kind of assurance system was developed e.g.in Germany based on a liability fund fed by the producers of sewage sludge able to compensate even for unknown risks caused by sludge application. Today the greatest economic risk for farmers using sludge is not with reduced crop quality but with the marketing argument that crops grown without sludge are of better quality and therefore yield higher prices on the market. Sludge disposal options can also be discussed using the following precautionary criteria: • no accumulation of conservative compounds (heavy metals, organic micro-pollutants) in soils, plants and crops; • material input to soils and output have to be equal; • no observable effect for soil biocoenosis, crops and their consumers, • no deterioration of ground water quality. These criteria can only be applied using models describing all inputs and outputs to and from the soil as well as all transformation processes. Such models contain important uncertainties and it is very difficult to asses the associated risk. As a consequence of the situation described above a rational decision whether sludge application on land according to “best practice” is good or bad cannot be expected. This is in favour of incineration or other processes with similar effect as the most reliable solution for sludge treatment and disposal of the ashes. The following article tries to show that the problem of sludge disposal is often overestimated in terms of ecologic and economic relevance at least on a national level, even more on EU level. It is the goal to develop a methodology and several indicators enabling decision makers to assess the relevance of the sludge disposal problem in connection with agriculture on a national level. REFERENCE VALUES FOR QUALITY AND QUANTITY DATA In order to make data comparable between different locations and on a different time scale it is essential to find reference values. For waste water and sludge there must by a close relation to the pollution load caused by one person (inhabitant). Also industry and trade are producing pollution resulting in sludge production. Summarizing it is possible to derive two reference values: one person (p) connected to the treatment plant and one population equivalent (PE) corresponding to 60g BOD5/d as pollution load in the treatment plant influent. The ratio between population equivalents (PE) and inhabitants (p) ranges from 1 (small communities) to more than 2 (larger cities). Data on the actual BOD5- loading of all Austrian treatment plants show an average value of ~2 PE/p.
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The area used for agricultural production can be used as a reference for national agriculture. This area can be related to the inhabitants if it is sufficient to nourish its population. The dosage of sludge is normally related to the area of one hectare and the same holds for fertiliser application, crop yields etc. In this report 1 ha of agricultural land is used as reference value. In Austria there are ~4 Mio ha of agricultural land which is 0.5 ha per inhabitant, or 2 inhabitants can be supplied with food per hectare. In Austria agricultural production approximately corresponds to the national food and feed consumption. This assumption is acceptable for rough calculations. 1 year is chosen as a reference period. For agriculture as well as for waste water treatment one year comprises all the seasons and is a suitable period for mass balances. Energy considerations are related to the primary energy consumption per inhabitant (p). Entropy level is not considered because it would complicate very much the considerations. In central Europe (D, F, A) the average primary energy consumption is in the range of ~50 MWh/p/year which corresponds to a mean power consumption of 6 kW/p. RELEVANCE OF ORGANIC MATTER (DRY SOLIDS) IN SEWAGE SLUDGE 50 to 60% of DS in the sludge is organic matter. It represents the largest valuable mass fraction of the dry solids. Sludge composting reduces this fraction. Maintenance of a good soil structure requires organic matter. The amount depends on the local climatic conditions and the crops cultivated. At Austrian climatic conditions about 2 t oDS/ha/year are needed. Table 1: Relevance of organic matter (dry solids in sewage sludge, situation in Austria)
The actual potential is < 1% as only 90% of the population is connected to central treatment plants, part of the sludge is composted before land application, about 15% are put to landfill after biological stabilization and ~ 37% of the sludge is incinerated. Consequently organic matter for soil stabilisation has to be produced by agriculture; sludge can only be relevant to cover local deficiencies. RELEVANCE OF SEWAGE SLUDGE NITROGEN COMPOUNDS IN AGRICULTURE Nitrogen is an unlimited resource. Market N-fertiliser production needs energy (10 kWh/kgN). Sludge normally contains only less than 20% of the nitrogen load in the waste water (Tab. 2). If nitrogen recycling is aimed at, urine separation (as suggested by e.g. GUJER, OTTERPOHL) is more favourable. Urine has a maximum potential of substituting about 55% of waste water N-load (~ 2.2 kgN/p/year) of market N-fertiliser, corresponding to an energy saving of ≤ 22 kWh/p/year or ~3 W/p. This is close to the energy consumption of modern nutrient removal treatment plants with sludge digestion and the use of the biogas for electric power production (NOWAK, 2003). The Austrian loss of nitrogen compounds via surface waters is ~90.000 tN/year (ZESSNER 1999). This is the same order of magnitude as market fertiliser application. N-losses in agriculture are 6 to 10 fold higher than in the waste water systems.
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Table 2: Relevance of sewage sludge nitrogen compounds in agriculture
RELEVANCE OF SEWAGE SLUDGE PHOSPHORUS COMPOUNDS IN AGRICULTURE It is estimated that the economically relevant global phosphorus ore reserves will be exhausted within a period of 75 to several hundreds of years depending on the assumptions made. Therefore P can be regarded as a limited resource As phosphorus fertiliser is vital for food supply of the still growing global population and cannot be substituted a reasonable P-management will be necessary in the future. Requirements for P-removal at treatment plants therefore are not only relevant for eutrophication abatement but also from P recovery. Table 3: Relevance of sewage sludge phosphorus compounds in agriculture in Austria
One person discharges about 2g P/day to the waste water, i.e. 0.7 kgP/p/year. Industry and trade additionally contribute to the P-load reaching municipal treatment plants. P load in sewage sludge can vary in a broad range from about 0.4 to 1.5 kg P/p/year depending on P-removal standards and the use of phosphate free detergents. All attempts to recover “clean” P from sewage sludge have not been successful up to now in regard to cost efficiency (ROELEVELD et.al. 2004), P recycling can be achieved by sludge application in agriculture. Incineration with mono landfill results in P-stocks with high P-concentrations (up to ~6% P, ~14% P2O5) for future generations. Plant availability of phosphorus in sludge (or in ash) is strongly depending on treatment processes applied and has to be considered if applied in agriculture.
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In Austria P-free detergents prevail and there is a P-removal standard (0.5 to 1 mgTP/l) for all treatment plants >1000 PE. If P fertilisation is reduced to the minimum requirement in agriculture sludge could substitute ~40% of P-market fertiliser imports. This potential could become very relevant for future P-management in Austria. In Austria a long term average sludge application of about 1t DS/ha/year is recommended (OEWAV 2004). Table 3 shows that the loss of phosphorus via the surface waters in Austria is nearly equal to the phosphorus transferred to sewage. From the sustainability point of view P in sewage sludge is relevant. RELEVANCE OF SLUDGE TRANSPORT Sludge transport load mainly depends on the water associated with the solids and varies from ~10 to 500 kg/p/year. Transport distance and load determine the energy consumption for transport. Even sludge transport seems to have low relevance in developed countries it can have a relevant impact on a local or regional scale and for comparing different sludge disposal solutions. Table 4: Relevance of sludge transport
RELEVANCE OF ENERGY CONTENTS IN SEWAGE SLUDGE Table 5 shows the total energy content of waste water pollution and the sludge related to p and PE. The calculations are based on a COD/BOD ratio of 110/60. As mentioned above table 5 does not reflect entropy i.e. the value of energy recovery, which is highest in methane from sludge digestion and is lowest in waste water. The influence of the water content of the sludge on energy recovery is also not considered. The same is with the thermal energy contained in waste water. From table 5 it can be concluded that sludge treatment and disposal is very relevant for energy management at the treatment plant but will not be relevant for national energy supply in developed countries (
