Food Waste Disposers

Introduction

The Chartered Institution of Water and Environmental Management (CIWEM) is an independent professional body representing over 12,000 managers, and other professionals, in all sectors, who are responsible for the stewardship of environmental assets. CIWEM's agreed purpose is to develop and promote better and integrated management of the environment; to foster a deeper understanding of water and environmental issues and to enhance the quality of people's lives. This is achieved through CIWEM's Royal Charter, education, training and professional development; dissemination of information; conferences and events; research and publications; contact with Government agencies and other bodies, partnerships with other organisations and the publication of Policy Position Statements (PPS).

Purpose

To outline the main issues relating to the use of food waste disposers (FWD). These are installed beneath sinks to macerate food waste in order that it can be disposed to the wastewater collection and treatment system as an alternative to disposing it with solid waste. The issues include the effect of food waste on the wastewater system, the diversion of food waste from landfill and consequent reduction in methane production, avoidance of extra vehicle movements for separate collection, avoidance of vermin attraction and avoidance of storing putrescible food waste in or close to kitchens with its associated health and odour implications.

Background

FWD were introduced to the market in North America in the 1930s. North America has the greatest density of domestic installation at about 50% of households. This is an order of magnitude more than any country in the EU, where food waste is generally disposed as part of the solid waste system. However the density of installation in commercial kitchens is very much greater.

In the EU the Landfill Directive¹ requires reduction in the amount of biodegradable waste disposed to landfill. The environmental objective of this requirement is to reduce methane emissions from landfills. It is estimated that methane has more than 20-times the climate change effect of carbon dioxide over 100 years. Several countries and municipalities have introduced separate collection schemes whereby the solid waste from domestic and commercial premises is required to be stored in separate containers, collected separately and taken to treatment facilities. The biodegradable fraction is generally composted or (less often) anaerobically digested, with the methane used as a renewable energy source. Separate collection often necessitates extra truck traffic, especially during summer when it is not acceptable to store biodegradable waste for long periods prior to collection because of odour.

The question of the place of FWD in EU waste policy was raised in 2001². This document said "In order to avoid an unjustified increase in the quantity of sewage sludge, it should be prohibited to shred solid biodegradable waste with a view to evacuating it via the sewer." It raised the question of the relative environmental benefits and costs of FWD compared with separate storage and collection.

The Working Document² carries the introductory rider "This working document is intended as a basis for preliminary discussions. It represents the opinion of DG ENV.A.2¹ only and does not necessarily engage the Commission." The Director of Sustainable Development has subsequently confirmed (Priv. Comm. 2001) that an environmental directive is not able to ban equipment such as FWD but that Member States or municipalities could ban them if they had grounds to believe that they jeopardised sewerage or wastewater treatment.

Several States and municipalities have banned the installation of FWD but these bans appear to have had no objective basis because when the case has been examined objectively bans have been reversed. For example FWD have been banned in New York City since the 1970s but this was rescinded in 1997 following a 21 month study by the Department of Environmental Protection (DEP)³ of 3 paired groups of different types of apartments (514 with FWD and 535 controls). The DEP modelled capital and operating cost implications (sewerage, wastewater treatment, and solid waste disposal) and concluded that overall there was no significant difference.

In Sweden the town of Staffanstorp has been studied⁴ and concluded that in several cases FWD provide a very good solution to the waste problem. No accumulation in drains or sewers was found, neither was there a change in water consumption. The change in wastewater treatment, biogas and biosolids offset the solid waste collection. The nutrients from the ground food waste improved biological phosphate removal.

There have been similar conclusions in other studies, for example in the Netherlands⁵ , Germany⁶ and Israel⁷ . Approximately 33% of ground organic kitchen waste solids were solubilised and the remainder were transported evenly as bed load and as suspended solids even at low flow velocities and in the low sewer gradients common in the Netherlands. There was no impact on fat accumulation. It enhanced biological nutrient removal and increased biogas production at wastewater treatment works and reduced the amount and moisture content of municipal solid waste.

The studies cited above (and others) indicate that domestic FWD do not prejudice the performance of wastewater treatment works but there have been instances where FWD in the kitchens of commercial residential establishments have overloaded the small rural works to which they are sewered. This was because of lavish over-provision of food and consequent waste. In the UK regulatory control of this situation is the prerogative of the Local Authority rather than the sewerage undertaker. Fortunately this situation is the exception rather than the rule and soluble by tripartite discussion.

Many sewers carry rainwater (especially in older systems) and even where surface water connection is not intended there can be infiltration that swells flows in wet weather. Sewer capacity cannot be infinite and therefore it is sensible to make provision for overflow in the most severe conditions where it causes the least inconvenience. The Environment Agency normally requires that solids greater than 6mm in any direction are removed from Combined Sewer Overflows (CSO) and that sewage is not macerated upstream of a CSO. Sewage contains hair and other fibres that form a mesh across screens; finer solids can accumulate on this mesh. CSOs are generally raked mechanically to prevent such accumulation, which would lead to blockage if it were not removed. Installation of FWD increased the per capita suspended solids by 33%⁴. Measurements of the output from FWD show that about 98% of the input is reduced in size to <2mm⁶ therefore the solids cannot themselves block a CSO. However FWD do add fine particles to the general load in sewage, which would be discharged to the aquatic environment in the event of overflow. If the CSO design is such that particles <2mm accumulate on other debris and result in blockage, the impact of FWD will be to reduce the time to blockage rather than determine whether it will block or not.

Another consideration is the risk of veterinary disease transmission as a result of contaminated meat being disposed. One could argue that meat in kitchens should by definition be fit for consumption, but this neglects illegal meat imports. The evidence of the limited surveys of incoming flights and other routes suggests that there is considerable illegal meat importation. It is suggested to have been the cause of the recent swine vesicular and foot and mouth disease outbreaks. However DEFRA considers that, even with more stringent controls at points of entry, it will be impossible to eliminate illegal meat imports entirely. Fortunately the disease agents have a similar susceptibility to wastewater and sludge treatment as the other organisms of concern. Voluntary changes in practice by the water companies⁸ mean that even if infected meat were disposed via FWD to sewer the disease agents would be subject to two control barriers to veterinary infection. The first barrier is sludge treatment (conventional or enhanced) and the second the land use restrictions set out in the matrix⁸. CIWEM welcomes and celebrates the voluntary agreement and that government is going to use this as a "bottom-up" approach to revising the relevant legislation for use of biosolids on land.

The final consideration is financial. It is clear from the studies cited above that removing food waste from the solid waste stream reduces the cost of collecting and disposing that stream, however it is transferred to the liquid waste stream. If the receiving wastewater treatment works has anaerobic digestion (AD) and derives income from the biogas, the additional biogas more than offsets the cost of treating the aqueous stream⁶. The additional biogas could even make the difference between viability and non-viability of gas utilisation. But if the works does not have AD, or if it is unable to use the biogas there is nothing to offset the additional treatment cost. In many countries the local authority is responsible for both liquid and solid waste and cost transfer is not really critical. However in the UK liquid and solid wastes are completely separate businesses and it would be equitable to reimburse the wastewater business for relieving the solid waste business of part of its costs.

Key Issues

1. It appears that regulatory restrictions on the use of FWD have been the result of prejudiced opinion rather than objective assessment and that where impacts have been assessed objectively there has been shown to be no case for such restrictions.
2. Especially in areas of high-density accommodation the case for FWD as an alternative to separate collection of biodegradable waste appears strong, on the grounds of avoiding additional traffic and odour and possibly disease. Disease vectors such as foxes, rats, birds and flies are attracted to large particles of food in solid waste but FWD solubilise and grind food so that it is no longer attractive to these creatures.
3. The change in water usage associated with operation of FWD has been measured to be trivial or not significant.
4. The risk of accumulation in sewers has been found to be non-existent because the specific gravity of the ground waste is so low that the particles remain in suspension. Fat has been found to sorb onto other particles and therefore to be conveyed with the wastewater rather than to deposit on the sewer walls.
5. FWD do not alter the risk of blockages at CSOs with 6mm screens because <1% of particles are >5mm, but they might reduce the time to blockage for those that are prone to accumulate particles <2mm diameter.
6. There is currently understandable concern about veterinary risk in general, but since FWD will in the main process materials deemed fit for consumption their use should have no impact on this risk. Even if unfit, illegal meat were processed, the dual barriers of wastewater and sludge treatment and matching of biosolids type to land use will provide adequate control. In the event of overflow, the disease risk from food waste is unlikely to be as great as from faecal material, given that people disposing illegal meat are also likely to have consumed it and if there were veterinary pathogens they would be in consumers' faeces.
7. The increased oxygen requirement for secondary wastewater treatment has been found to be more than offset by the additional biogas production at sites where there is anaerobic digestion.
8. The additional food value of ground organic kitchen waste can improve biological nutrient removal (BNR) at wastewater treatment works. [Food supply is performance-limiting factor for BNR at some works.]
9. The addition of ground kitchen waste (high in organic matter, fat and moisture) to wastewater increases biogas production at works that anaerobically digest sludge. This has been estimated at 300MJ/resident·year⁶ (equivalent to 8 litres diesel). It also reduces the moisture content of the residual solid waste, which increases its calorific value and/or makes it easier to separate into useable fractions.

Conclusions

1. CIWEM recommends that policy decisions be proportionate to risk and based on objective assessment and that this applies as much to FWD as to other issues. Where there is insufficient information to allow objective assessment, action should be put in place to fill the information gap⁹ .
2. CIWEM considers that FWD may have a useful place in the management of food waste and that they might be a more convenient and environmentally superior alternative to separate storage and collection. However there is the question of cost transfer.
3. CIWEM would like to see additional independent research to further expand the growing body of evidence about FWD.

References

¹ Council of the European Communities (1999) Council Directive on the Landfill of Waste, 1999/31/EC
² European Commission Biological treatment of biodegradable waste, DG ENV.E.3/LM/biowaste/1st draft Brussels, 20 October 2000.
³ New York City DEP (1999) The impact of food waste disposers in combined sewer areas of New York City.
⁴ Nilsson et al. (1990) Waste management at the source utilizing food waste disposers in the home; a case study in the town of Staffanstorp. Dept. Environmental Engineering, University of Lund.
⁵ De Koning, J. And van der Graaf, J.H.J.M. (1996) Kitchen food waste disposers, effects on sewer system and wastewater treatment. Technical University Delft.
⁶ Kegebein, J.; Hoffmann, E. and Hahn, H.H. (2001) Co-Transport and Co-Reuse, An Alternative to Separate Bio-Waste Collection? Wasser. Abwasser 142, 429-434
⁷ Galil, N.I. and Yaacov, L. (2000) Integrated solid waste systems including domestic garbage disposers. Proceedings CIWEM, Aqua-Enviro 5th European Biosolids & Organic Residuals Conference.
⁸ Safe sludge matrix. www.adas.co.uk/matrix
⁹ Commission of the European Communities (2000) Communication from The Commission On The Precautionary Principle COM(2000) 1 final Brussels, 2.2.2000

February 2003

Note:- CIWEM Policy Position Statements (PPS) represents the Institution's views on issues at a particular point in time. It is accepted that situations change as research provides new evidence. It should be understood, therefore, that CIWEM PPSs are under constant review and that previously held views may alter and lead to revised PPSs.