Iwa international Specialist Conference

2.

Sustainable sludge management – what are the challenges for the future?

Wim Rulkens
Developments in municipal wastewater treatment during the last twenty years are characterised by two aspects. The first is a continuous effort to improve the quality of the effluent by upgrading existing treatment plants and designing new more effective plants. The second is an increasing awareness of the problems associated with the sewage sludge produced in the treatment process: high costs and risks to environment and human health. Due to these aspects an increasing growth into the research on sludge treatment processes is observed. However it is remarkable that the waterline is rarely involved in this research although the waterline processes strongly influence quality and quantity of the sludge. Also the sustainability is very often not an issue of high priority in sludge management.

The aim of this paper is to give a structural approach on how to achieve a more sustainable sludge management strategy in the future. Starting points are an integral approach of both the waterline and sludge line and the approach to recover and reuse valuable products as much as possible. Current sludge treatment processes as well as possible innovative processes are discussed. The paper has the intention to provide a framework that can be used in the discussion about sustainable wastewater and sludge treatment in future.

To identify potential scenarios for a more sustainable sludge treatment it is useful to briefly assess the composition of the sludge. Very indicative this composition is characterised by five groups of components: a. Organic carbon, phosphorous, (Kjeldahl-N); b. Toxic inorganic and organic micropollutants such as heavy metals, PAHs, and pesticides; c. Pathogens; d. Inorganic compounds such as silicates and aluminates; e. Water. The fundamental problem of sludge is that all these compounds are present in one mixture. Organic carbon, phosphorous, and nitrogen are valuable compounds. Often, this holds for the inorganic compounds as well. Sustainable treatment involves the recovery and useful reuse of the valuable products and the destruction of the toxic compounds.

There are six basic approaches with the potential to tackle the sludge problem in a more sustainable way.

The first approach is to improve directly or indirectly the quality of the sludge, for example with respect to heavy metals. Within this approach three options are possible. a. Prevention of the discharge of toxic micropollutants. Over the last thirty years a strong decrease in the discharge of pollutants is observed and it is likely that also in the future this decrease will be continued; b. Removal of colloidal and suspended particles from the influent as a first treatment step. With exception of nickel, heavy metals in sewage are mainly bound to the particles present in the wastewater. Removal of these particles results in a primary sludge that is relatively strongly polluted with heavy metals. The remaining effluent is free of heavy metals and can be treated biologically resulting in a biological sludge of good quality; c. Removal of the heavy metals from the sludge by chemical leaching with inorganic and organic acids, complexing agents or by microbiological leaching using Thiobacilli. The mentioned methods are not or less effective for removal of toxic organics or pathogens.

The second approach is the beneficial use of organic carbon. Within this approach four options are available. a. Increased production of biogas as an energy source by pretreatment of the sludge. Possibilities are hydrothermal heating, microwave heating, ultrasonic heating, use of ozone, use of enzymes, use of liquid jets, pretreatment (hydrolysis) with sodium hydroxide or wet oxidation; b. Production of a fuel (diesel oil, gaseous products from pyrolysis, gasification or hydrothermal heating); c. Use as an energy source in case of application of thermal treatment processes such as melting and slagging technology, the use of dried sludge in cement kilns (where also the inorganics are utilised) and the use of dried sludge as an energy source in, for example, coal fired energy power plants. d. Production of volatile acids such as formic, acetic and propionic acids.

The third approach is the reduction of the total amount of sludge either by the conversion of carbon or by the removal of water. Additional to processes already mentioned, there are biological treatment in combination with ozone treatment, use of higher organisms (such as protozoa and metazoa), aerobic and anaerobic composting, and vermicomposting. Advanced dewatering processes which are already available or still in various stage of development are electro-osmotic dewatering, advanced drying processes, conditioning by freeze-drying, and Carver Greenfield evaporation. In case of a dewatering process the aim can also be the increase of the thermal value of the sludge.

The fourth approach is the recovery of phosphorous (P) for reuse, for example as fertilizer. There are three options to recover P. a. P recovery from the sludge by thermal, chemical or microbiological methods or combination of these methods; b. P recovery directly from the waste water; c. P recovery from the ashes of sludge incineration plants.

The fifth approach is based on a change in treatment scenario of municipal wastewater. There are two possibilities. One possibility is a centralised treatment system based on physical-chemical wastewater treatment steps. The first step is a physical chemical treatment step focused on complete removal and concentration of colloidal and suspended particles. The subsequent step is use of a hyperfiltration step in which soluble P, Kjeldahl-N (mainly NH₃) and organic carbon are concentrated, making it more easily to recover P and Kjeldahl-N by precipitation. The remaining concentrate, containing soluble organic carbon compounds, is digested together with the sludge from the first step. In this way, nitrogen and phosphorous and the permeate of the hyperfiltration process are more or less completely available for beneficial use. The second scenario is decentralised sanitation. A strong point of this scenario is that no dilution of wastewater takes place, making it more easy to recover value products for reuse. Also large transport systems are not necessary. Weak point is that an infrastructure is necessary to collect products. Furthermore, costs advantages of large scale treatment are missing. Another concern is potential risks for human health.

The sixth approach is in fact a combination of two or more of the previously mentioned approaches.

An assessment by means of a multi-criteria analysis is necessary to select the approach which is most optimal with respect to sustainability. Main factors which have to be assessed are: a. Technical feasibility; b. Economic feasibility; c. Environmental feasibility, more specifically defined in terms of energy use, prevention of emissions of pollutants and waste, and recovery and reuse of valuable products also including the treated water; d. Marketing aspects of the products; e. Infrastructural and logistic aspects; f. Environmental policy and legislation; g. Implementation route; h. Potential risks for human health; i. Public acceptance; j. Future technological developments in other areas of pollution control.
It is evident that the way how to achieve a sustainable development may not only vary between highly industrialised western countries and developing countries, but also between individual countries. There is no such thing as a uniform and unique system which is most sustainable. Each situation basically asks for its own tailor made solution which is most sustainable to the conditions of the specific area in question. This need for diversification is a characteristic aspect of sludge management. Due to the many factors influencing the sustainability of a sludge system, but also due to progress in insight in the problem and progress in technological tools, it will be clear that sludge management has to be considered as a dynamic activity.

(Wim Rulkens) 02-0077

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ABSTRACT

COMMUNITY PARTNERSHIP IS THE WAY FORWARD

PROF PETER MATTHEWS

BOARD MEMBER ENVIRONMENT AGENCY ENGLAND AND WALES

 

The UK has been operating Codes of Practice  for the use of sewage sludge as biosolids , with growing complexity since the 1970' s . There has been increasing recognition of the role of all community stakeholders - not just the users of the product .Whilst national wate disposal policy places  avoidance  and minimisation first , there is a focus on using the waste products as a resource . However, in the past , Regulators have tended to take an arms length approach to the decisions of disposers and only regulated the disposal activity . In January 2000 the Environment Agency launched its Vision for the Environment and a Sustainable Future . In delivering its part of the Vision , nine themes were identified . Its focus is in outcomes in which a wide variety of tools and techniques will be employed .Regulation will be just one part of these . One theme was the environmental outcome of restored protected land with healthier soils .Another theme was to seek wiser sustainable use of natural resources . Waste is to be considered as a potential resource with the efficient reuse and recycling as the social normI .Imaginative approaches are to be used which will change attitiudes and behaviour and the Agency declared that it wants to be  a trusted partner in collaboration with all stakeholders

 

Quite appart from the growing public awareness of waste issues and concerns about  environmental impacts , there is also heightened attention to food quality following the BSE and foot and mouth disesase events . The Agency and other stakeholders in biosolids management are mindful of the effects of new European legislation which will restrict opportunities for managing waste and in the case of the interest of this paper , sewage sludge disposal.To aid this challenge and as a clear practical example of the Vision in practice , the Agency launched a pilot project to determine if a partnership , called pro-tem the Organic Resources Recycling Partnership would be welcome and practical . The indications are that the answer is most emphatically - yes !!. The objects will include support for  biosolids use . By the time of the conference the Partnership should be launched and this will be of real benefit in applying new Regulations due in 2003 and for which consultation is to be launched in late October .

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Route to synthesize the sludge management process
D. J. Lee

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 10617, [e-mail]

An “optimal” sludge management is expected to deal with individual sludge problem at minimum cost, taking into account the risks and benefits by sludge disposal, and getting wide public acceptance of the practice. However, the selection of an appropriate system for sludge management depends on many factors, most of them based on local circumstances. Any decision should have, as a priority task, the maximization of material and energy recoveries from sludge, and the minimization of the total energy needs and cost of processing, but other important factors, such as local geography, climate, land use, economic situation, regulatory constraints and public acceptance of the various practices, must be accounted for.
The success of sludge management depends on an initial choice of a cost-effective and sustainable process based on local and site-specific considerations. Restated, on the basis of the constraints by regulations and local geometry, climate, and technical and economical situation, an “optimal” treatment process could be “synthesized” with optimization goal, such as least cost/risk, or sustainability. This talk will present a systematic way to synthesize the optimal disposal/use practice of sludge with known sources and quality, by defining all treatment stages as a unit of the entire system, the material flows connecting them, and employing a nonlinear programming scheme. The flexibility of the synthesized process subjected to future regulatory change or sludge production increase will be analyzed.

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PATHOGENS IN SLUDGE: A CASE OF SUFFICIENT CHALLENGE

P. Aarne Vesilind

Department of Civil and Environmental Engineering

Bucknell University

Lewisburg PA

USA

In H. G. Wells’ classic story The War of the Worlds, written in 1898, we humans are about to become domesticated farm animals to feed the Martians, new masters of the Earth. In a wonderful exchange of views by two humans hiding out in the sewers, the debate ensues on whether it would not be too bad to be taken care of like a farm animal and periodically harvested, with our hero holding out for human dignity. Before they both becomes hamburger meat for the Martians, however, the world is saved by an unlikely ally. The Martians had not counted on the presence of pathogenic microorganisms on Earth, and lacking immunity against these pathogens, succumb to the attack by our little friends. The Martian's lack of immunity to pathogens was due to the lack of pathogens on Mars (according to Wells), and thus they had not had an opportunity to develop the immune systems necessary to fight off such attack.

The historian Arnold Toynbee concluded that no great civilization has risen without a sufficient, but not overwhelming, challenge from its physical environment or from its neighbors. Toynbee cites many civilizations that were unsuccessful because of the lack of sufficient challenge or because of the presence of an overwhelming challenge such as a more powerful neighbor.

The toxicologist H. F. Smyth Jr. has used this concept to describe an apparent anomaly in toxicological measurements: animals in the group with the lowest exposure to test chemicals or biological pathogens grow more rapidly, have better general appearance, live longer, and have less health problems such as tumors, than animals in the control group that receive no exposure to the toxins. Smyth uses Toynbee's term, "sufficient challenge", to describe the beneficial effect of low exposure to toxins.

The concept of "sufficient challenge" is brilliantly demonstrated in the debate over setting pathogen and chemical standards for sludge use and disposal. As more and more nations adopt such standards, and existing laws are reviewed, it is useful to consider the scientific underpinnings for such regulations. The objective of this paper is to review the method and wisdom used in setting such standards.

The paper begins with a review of the existing evidence for the effect of sludge disposal on land, relying especially on two extensive reviews performed by the National Research Council of the United States. [Use of Reclaimed Water and Sludge in Food Crop Production, 1996, and Biosolids Applied to Land, 2002] Both studies conclude that there is no evidence that proper use of wastewater treatment sludge on land has any detrimental effect on either the people working at the site, on the population surrounding the land application site, or on people eating the crops grown in the sludge-amended soils. Nevertheless, it is well known that wastewater sludge contains all possible human pathogens and thus represents a potential public health problem, and both studies recommend (predictably) that more information is needed to close the gap between what the authors see as a source of pathogens and what they intuitively feel must be a potential public health problem.

In this paper, I argue that seeking such information is like trying to prove a negative. Can there be an incidence of human disease related to sludge disposal? Other than acute cases of improper sludge management which can occur, I argue that it is highly unlikely that we will ever obtain information on the detrimental effect of sludge management, simply because low exposures to such pathogens might actually enhance human health. Coming into contact with small doses of pathogens is the "sufficient challenge" our bodies need in order to stay healthy. Clearly an "overwhelming challenge", such as an acute episode due to mismanagement, is to be avoided, but our enhanced health comes not from zero exposure, but a "sufficient" exposure to toxins.

This paper argues for a rational basis for setting sludge standards, based not on zero tolerance or a linear dose-repose curve, but on the concept of "sufficient challenge". It makes no sense whatever to impose stricter sludge disposal standards in the absence of a problem, and knowing that stricter controls may actually be detrimental to human health. We do not want to end up like the Martians, having no immunity to fight of pathogens when they do attack us.

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DEVELOPMENTS IN SLUDGE CHARACTERISATION:

THE EVALUATION OF SLUDGE CONSISTENCY