Iwa international Specialist Conference

IWA International Specialist Conference

BIOSOLIDS 2003-Wastewater Sludge as a Resource

Norwegian University of Science and Technology (NTNU), 23.-25. June, 2003

ABSTRACTS

1.

Wastewater sludge - the challenges

What are the potentials of utilising the resources in sludge?

Helmut Kroiss,

Vienna University of Technology, Institute for Water Quality and Waste Management

ABSTRACT

EU legislation (EU Directive 2717) as well as many national water acts in an increasing number of states worldwide has defined mechanical and biological treatment of urban waste water as a minimum requirement for receiving water protection. The consequence is an increasing sewage sludge production which causes a sludge management problem. Operators of waste water treatment plants do not produce sludge to meet a demand of consumers but as by-product of fulfilling legal requirements. As a consequence waste water treatment requirements can only be met if the sewage sludge can be reliably disposed off at any time.

Sewage sludge contains all compounds present in the effluent of a treatment plant and a fraction of the raw waste water compounds. Sludge can be classified as a sink of the urban material flow and therefore contains valuable and potentially hazardous material.

Table 1: Contribution of sewage sludge compounds to the regional anthropogenic mass flow

Regional flow	Mass	Dry matter	C	N	P	Cd
Total flow	100	100	100	100	100	100
Municipal solid waste	0,2	4	2	9	6	20
Sewage sludge	0,3	0,3	0,2	3	8	1

From Table 1 it can concluded that within the total regional material flows sewage sludge solids are of low relevance except for phosphorus, which becomes even more important (up to 15% of the total flow) if P-removal efficiency at the waste water treatment process reaches ~ 90%. In addition global mineral phosphorus reserves are limited. If the actual trend in commercial P-fertilizer application in agriculture holds on it is estimated that the world reserves are being exhausted within 70 to 150 years.

Cadmium as a typical representative of potentially hazardous heavy metals in the sludge also is not relevant in the regional mass flow and has strongly decreased due to improved source control.

Organic matter (~10kg/inhabitant/a) contained in stabilized sewage sludge can have only local importance at least at moderate climatic regions, as agricultural production of organic matter is several orders of magnitude higher.

Nitrogen has also low importance especially as long as nitrogen removal at treatment plants is applied. If the reuse of nitrogen from waste water flow is aimed at, urine separation technique seems to be the most favourable option. In any case nitrogen is an unlimited resource in the atmosphere.

The conclusion is that phosphorus is the most valuable compound in sewage sludge. The recovery of phosphorus can become a vital resource for food production of the global population in the foreseeable future. It can also be stated that conventional sanitation systems can reach the same phosphorus recovery efficiency as the newly developed separation systems for urine and faeces. Even if sludge is incinerated phosphorus can be recovered as it is transferred to ash and slug.

Sludge contains valuable compounds for agriculture as P, N, organic matter, etc., and in many cases the easiest and least cost reuse of these valuable compounds is the application of stabilized sludge on arable land with or without dewatering. Land application of sewage sludge in agriculture is still performed in many regions and there exist a variety of regulations for it. But in most of the countries this disposal route has not proved to be reliable, which means that all treatment plants will have to provide a second disposal option, too.

If the “beneficial use” of sludge in agriculture is not possible or feasible, other technical solutions for final sludge disposal as use for landscaping and recultivation, landfill disposal (after pre-treatment as e.g. incineration) can be applied. Industrial use of sewage sludge was never driven by a demand of the industry, in practically all cases it has been developed or applied because it was a feasible disposal method under specific local and historic situations. From the ecological point of view industrial use is doubtful, especially if the phosphorus in the sludge is “diluted” in products (cement, construction materials, ashes from co-incineration, etc.) from which phosphorus can not be recovered economically.

As long as sludge is not accepted as a product which can be sold on the market at competitive prices, sludge will be classified as a waste as it is defined in most of the waste management acts. “Waste” has inherent negative aspects in public perception. One of these aspects is that disposal has to be in conformity with waste disposal regulations in order to avoid hazards and causes costs.

Farmer unions and land owners tend to classify agricultural use of sewage sludge or products made from sludge (“soils”, composts, etc.) as waste disposal method for which they will have to be paid for. Food industry and retailers are afraid of the risk associated with the fact that at least today “healthy food” cannot be positively linked with “sewage sludge” and “waste” in public opinion, even sludge application in agriculture is subject to rigorous quality control.

Sludge treatment techniques are strongly determined by the disposal options. Reliability considerations and overall costs minimization play important roles, too.

The most efficient process in regard to the destruction of potentially hazardous organic compounds (pathogens, AOX, PCB, phenolic compounds, pharmaceuticals, endocrine disruptors, etc.) and in regard to mass minimization for disposal is incineration with adequate flue gas cleaning. Incineration requires optimisation of the sludge dewatering process in order to obtain self sustaining combustion avoiding the addition of low entropy fuels. Stabilisation processes before incineration can contribute to improved logistics (transport, storage) and odour prevention. They often do not contribute to improved energy management.

Many of the mechanical, physical and biological sludge treatment processes have been developed in order to reduce the organic fraction of the solids. They can be advantageous if dewaterability is not deteriorated, otherwise the mass of sludge to be transported and/or disposed off can remain the same or even increase. Their application has to be based on sound economic assessment. Some of these pre-treatment processes aim at meeting organic carbon limit concentrations for sustainable landfill disposal, too. But as nitrogenous compounds often are little reduced by these pre-treatment processes, long term monitoring of ammonia in the landfill leachates (up to 300 years) will be required for ground water protection, which is not a sustainable option.

Even dilution is illegal as a means of meeting limit concentrations agricultural use of sludge finally results in a dilution by the natural soil material. Many of the sludge compounds are transformed in this way from potentially hazardous to valuable trace compounds (nutrients, some of the heavy metals) or at least to background concentrations of the soils. Mixing sludge with inorganic material (sand, etc.) or with compost from other sources before land application does not reduce the hazard potential of sludge compounds. “Soil” production from sludge should be assessed from the economic and ecologic point of view taking into account transport, energy requirements, air pollution, manpower, long term effects on soils etc. and not primarily in regard to meeting actual concentration standards. Successful marketing strategies for sludge (biosolids, garden mould, “soil” etc.) are not necessarily in accordance with ecological considerations, but they have the potential to combine ecologic and economic feasibility. Marketing is closely related to confidence building, quality control of the products and services and continuous public relation management which has to include the media.

Conclusion is that sewage sludge can be regarded as a valuable resource as well as a potentially hazardous waste. In moderate climatic regions phosphorus is the most valuable compound to be recycled to the soils. Where P recycling by sludge application in agriculture is politically and/or economically not feasible and as long as P-recovery from sludge is economically not feasible mono landfill disposal of the end-product of sludge handling (e.g. incineration) has to be aimed at in order to have access to this P resource (up to about 8% P) in the future.

Sewage sludge composition is reflecting our lifestyle and is produced at the point of intersection between water supply and water protection, food supply and agriculture, between soil protection and health care, legal obligations and property rights. The solution of the sludge disposal problem will remain a political problem as it affects nearly the whole population as stakeholders. Scientific research, technology development and long term investigations can contribute to assess the risk of different disposal methods and to improve existing or conceive new materials’ management options. Politicians and engineers will have to be aware that waste water management has important psychological, sociological and economic aspects which always have to be included into the decision making on legal frameworks and technical solutions for the disposal of sewage sludge or other by-products.

Irrespective of sludge disposal route source control of all potentially hazardous compounds has a great priority for receiving water protection. Improved sludge quality is a most desirable by-product and quality control of the sludge an excellent tool for the monitoring of source control efficiency.

From economical and ecological considerations a variety of different solutions for reliable sludge disposal and safe reuse of valuable sludge compounds will have to be applied and developed also in future in order to adapt to the specific local and historic situations.