Recently a number of sewage treatment systems have been developed utilizing membrane filtration. Originally developed for food and pharmaceutical industries, membrane systems have now seen widespread large-scale application in desalination and water treatment plants. This technology relies on a microporous barrier (normally polymeric) in order to filter effluent on the basis of particle size (usually to \1 km). To reduce fouling, membranes are generally used in a cross-flow arrangement, with the bulk of the effluent flow across the membrane surface through which treated permeate is removed.22 The process has the advantage of being able to deliver disinfected effluent of uniformly high quality, but can have a significant cost disadvantage compared with conventional treatment. Nonetheless, as perhaps the only truly new sewage treatment process to have been developed in the last ten years, membrane systems have attracted considerable interest from water companies.
Two specific developments are worthy of mention: (i) use of membrane separation as an alternative to biological secondary treatment, followed by disinfection for coastal sites, and (ii) membrane bioreactor systems.
Membrane SecondaryTreatment. Screening and primary treatment is followed
21 G. Ladiges and R. Kayser, Water Sci. Tech., 1993, 28, 11/12, p. 315.
22 J. Murkes and C. G. Carlsson, ‘Crossflow Filtration—Theory and Practice’, John Wiley and Sons, Chichester, 1988.
by membrane separation to remove remaining suspended solids and achieve disinfection by physical removal of pathogenic organisms, including bacteria and viruses. Since discharge of soluble BOD is rarely an issue at coastal sites, membrane treatment can significantly reduce capital expenditure as biological treatment, settlement, disinfection, and outfall costs are decreased. However, operating costs are higher and overall cost effectiveness depends on site-specific factors.23 Of the systems available, those from Memcor24and Renovexx25 are now seeing full scale application.
Membrane Bioreactor Systems. Membrane bioreactors combine biological treatment and barrier separation stages in one treatment system. Use of membranes to replace settlement tanks allows the retention of high levels of biomass in the biological treatment stage (typically 15 to 20 g l~1 mixed liquor suspended solids). This provides a compact system, giving a very high quality disinfected effluent and lower waste sludge volumes due to thicker sludges than conventional proccesses. However, the current cost of membranes limits cost competitiveness to smaller scale systems where high effluent quality or restricted land area is a concern. The fully automated system developed by Kubota, in Japan, probably represents the forefront in advanced sewage treatment systems.26
6 Conclusions
The effective and efficient treatment of sewage and industrial effluent is essential to prevent damage to the environment. This has been recognized and enforced by both UK and European legislation. A wide range of solutions will be needed to ensure that the tighter regulatory requirements are achieved on a consistent and more economic basis. In the industrialized nations, improvements to existing assets and optimization of treatment processes will dominate developments in treatment over the next decade.
In the developing world there is a clear need for appropriate low-cost sewage treatment. In these countries the main future requirements will be for sewerage in cities, and the provision of preliminary, primary, and eventually secondary treatment. It is a sad reflection that, over a century on from the discovery linking disease to contaminated water, some 25 000 people die every day as a result of lack of clean water and basic sanitation. Most of these are children in the developing countries. It is estimated that two billion people, nearly half the world population, do not have clean drinking water.
23 G. Owen, M. Bandi, J. A. Howell, and S. J. Churchouse, ‘Economic Assessment of membrane Processes for Water and Wastewater Applications’, Proceedings of the ‘Engineering of Membrane Processes II—Environmental Applications’ Conference, Elsevier, Amsterdam, 1994.
24 F. Hudman, P. MacInante, A. Day, and W. Johnson, ‘Demonstration of Memtec Microfiltration for Disinfection of Secondary Treated Sewage’, Sydney Water Board, Memtec Ltd. and Department of Industry, Technology and Commerce, Vol. 1, May 1992.
25 G. J. Realey and J. Bryan, ‘Preliminary Evaluation of the Renovexx Microfiltration System at Berwick upon Tweed STW’, WRC Report UM1379, May 1993.
26 H. Ishida, Y. Yamada, M. Tsuboi, and S. Matsumura, ‘Submerged Membrane Activated Sludge Process (KSMASP)—Its Application into Activated Sludge Process with High Concentration of MLSS’, Proceedings of the 2nd International Conference on ‘Advances in Water Effluent Treatment’, MEP, BHR group publication, London, 1993, 8, p. 321.
The charity ‘WaterAid’, which is supported by the UK Water Companies, is helping to address this problem. In 1993 it provided £6M to the developing countries of India, Nepal, Kenya, Ethiopia, Uganda, Tanzania, Zimbabwe, Ghana, Sierra Leone, and The Gambia, for water supply and basic sanitation.
This vital work continues.
Acknowledgement
The authors would like to thank all the Wessex Water staff who assisted in the formulation of this article, viz.: Sam Allen, Fiona Bowles, Phil Charrett, Steve Churchouse, Simon Cole, Rikk Earthy, Keith Fitzgerald, Emma Letts, Andrew Randle, Mike Tarbox, and Peter Wratten.
The views expressed in this paper are those of the authors and do not necessarily represent those of Wessex Water.
K . W ES TL A K E
1 Introduction
The disposal of wastes to land has been the prime means of waste disposal since the evolution of man. Since the late nineteenth century, the volume and hazardous nature of wastes generated has increased considerably, and has led to the need for disposal to land specifically allocated for the purposes of disposal—landfill. Even today, the disposal to land is often poorly controlled and managed, especially in developing countries. This article will not focus on these
‘dumps’, but on properly managed and controlled landfills. Nor is there scope here for debating in detail the relative merits of landfill disposal and other waste management options and their role in integrated waste management, although these concepts will be introduced.
European Union (EU) policy on waste management is clearly ennunciated in the Fifth Environment Action Programme ‘Towards Sustainability’ [COM(92)23]1 issued in March 1992. The programme sets long-term policy objectives and intermediate targets for the year 2000. For ‘Municipal waste’ the overall target is the ‘rational and sustainable use of resources’, achieved through a hierarchy of management options, viz.
f Prevention of waste.
f Recycling and re-use.
f Safe disposal of remaining waste in the following rank order:
(i) combustion as fuel;
(ii) incineration;
(iii) landfill.
Thus, it is clear that the European Union views landfill as the final waste disposal option. This view has been translated into potentially increasingly stringent controls over landfill as identified within the draft Council Directive on the landfill of waste [COM(93)275].2 The potential impact and the requirements of
1 Commission of the European Communities, ‘Towards Sustainability: A European Programme of Policy and Action in Relation to the Environment and Sustainable Development’, COM(92)23, 1992.
2 Commission of the European Communities, ‘Proposal for a Council Directive on the Landfill of Waste’, COM(93)275, 1993.
the Directive on the landfill wastes within Europe are too lengthy to discuss in detail here. However, at the time of writing, some of the main requirements are that
f clinical wastes will be banned from landfill;
f leachate and groundwater will be monitored at least twice per year for at least 30 years from the date of implementation;
f leachate collection/drainage systems should be sufficient to ensure that no liquid accumulates at the bottom of the site;
f landfill gas will have to be collected and treated unless an environmental assessment determines that this is not required; and
f a conditioning plan identifying the measures to comply with the Directive should be submitted to the competent authority within one year of the implementation of the Directive.
At the time of writing, the future of co-disposal (see Section 3) remains uncertain. Latest reports3,4 suggest that co-disposal will be allowed in countries where currently practised subject to conditions which are to be met within five years of implementation of the Directive. Existing landfills, or those created between 1994 and the adoption of the Directive, will have to meet the required standards within ten years of adoption. However, the wording of the revised document appears to allow for different interpretations, and uncertainty still remains.
Requirements such as those above represent a significant step forward for a number of countries within the EU; according to a recent report by Environmental Resources Management (cited in Reference 5), the number of uncontrolled landfills in Italy and Portugal exceeds 60% of the total number of sites, while in Greece and Spain the number of uncontrolled sites represent approximately 30%
of the total. Strict implementation of the Landfill Directive could result in a decrease in landfill capacity in Europe as sites opt for closure rather than continued operation under new specifications that for many would be both difficult and expensive to achieve.
In those countries whose landfill operations are better controlled, the impact of the Directive will still be significant; in the United Kingdom (UK) there are approximately 4000 licensed landfill sites where approximately 85% of controlled wastes and 70% of hazardous wastes (equivalent to approximately 2 million tonnes annum~1) are disposed. The proposed ban on co-disposal will, according to the Department of Environment (DoE),6 add an extra £160M year~1 to UK industries’ waste disposal costs. As a result of engineering and other requirements of the Directive, the cost of landfill disposal can also be anticipated to increase significantly. As the cost increases, and the differential between landfill and other disposal options such as incineration decreases, so the easier it becomes to use alternative disposal routes that are more favourably placed in the waste treatment hierarchy. In this way, the objectives of the EU Fifth Environment Action Programme begin to become achieved. Also, although cost is obviously
3 S. Tromans, Wastes Manage., 1994, July, 16.
4 Ends Rep., 1994, 233, 34.
5 Warmer Bull., 1994 41, 2.
6 Ends Rep., 1994, 228, 38.
an important factor, in countries such as the Netherlands, Denmark, and Japan, where local geology cannot support landfill as easily as in the UK, there is also a greater political will to find alternatives to landfill.
This review will examine the science, engineering and control of landfill disposal in the light of the above influences, using UK practice and control methods to highlight changing trends in landfill policy and practice.