Elsevier

Ecological Engineering

Volume 28, Issue 1, 1 November 2006, Pages 44-54
Ecological Engineering

Possibilities and barriers for recirculation of nutrients and organic matter from urban to rural areas: A technical theoretical framework applied to the medium-sized town Hillerød, Denmark

https://doi.org/10.1016/j.ecoleng.2006.03.009Get rights and content

Abstract

We aimed to develop and evaluate technical solutions for increasing recirculation of nutrients and organic matter from urban areas. Furthermore, we assessed the feasibility of the solutions in a specific medium-size modern town setting – the municipality of Hillerød (approx. 27,000 inhabitants), Denmark. Fourteen integrated systems were described based on well-known technology and assessed on the basis of their ability to meet requirements in relation to public health, comfort, costs, durability, user friendliness, energy use, and recirculation potential. Of these 14, four were used to ‘retrofit’ into the existing town in an integrated municipal sewage and organic waste management system and the resulting change in terms of energy balances, recycling, and economic costs were modeled against a conventional ‘reference’ waste management system. For the chosen systems the energy saving corresponded to the yearly electricity consumption in 896 Danish households. It was estimated that capital and running costs were approximately 10% and 17% higher than for the reference system, including the net energy savings. The reference system produced sufficient nutrients for fertilising 152 ha of agricultural field, whereas the alternative system in which nutrients were collected as sludge from biogas digester, human urine, and sludge from the wastewater treatment plant were enough for fertilising 451 ha of agricultural land.

On the basis of this work it can be concluded that it is technically possible to design integrated ecological waste management systems, based on known technical components that may be operated at or close to the cost level of the current conventional sanitation systems. Such integrated systems need to be further developed and tested on a scale that will allow an interaction with farmers and their organizations, in order to evaluate their wider environmental consequences and to further integrate health and socio-cultural aspects. The main drivers for change in countries with a fully developed centralized sewage and waste infrastructure would seem to be either government policy-motivated (sustainability drive) or motivated by grass root movements, since the economic incentives for technology change are currently uncertain and probably small. However, in the middle-income countries, including part of the developing world, there is potential for considerable cost savings, and enormous need for development of waste and sanitation infrastructure, which would give impetus to more effective use of ecological sanitation.

Introduction

Previously, in large parts of Europe and many other parts of the world, peri-urban farmers were dependent on deliveries of ‘night soil’ from urban areas in order to replenish the fertility of the land. In Japan, the recycling of urine and feces was introduced in the 12th century and in China, human and animal excreta have been composted for thousands of years. Such recirculating systems can still be found in parts of Asia, but are deteriorating in the face of the rapid development process, not least in the mega-cities that draw heavily upon the natural resources in their hinterlands. Thus, Færge and Magid (2001) assessed that for Bangkok only 7% and 10% of the N and P in the total food supply was recovered for agricultural use, the main part of the N being transported into the surrounding sea but with a large part of the P accumulating in the metropolitan area. In an ecological analysis this cannot be sustainable, since apart from the energy costs in many cases farmers in the hinterland cannot replace the nutrients they export to the urban areas with their products by mineral fertilizer, and the surrounding sea is becoming increasingly polluted.

In Northern Europe today, wastewater management systems have developed to maturity without primary concern for recycling (Magid et al., 2001). These systems were originally designed to get rid of waste in order to better the local hygienic standard using water as a transport media. More recently, environmental concerns have been the driving force behind a technological development of sewage treatment with biological removal of N, P and organic matter, thus increasing the costs for wastewater treatment. This technology addresses some immediate problems in the aquatic environment, but the sewage sludge from the treatment plants contain considerable quantities of xenobiotic compounds and heavy metals, and only a small fraction of the nutrients that entered the urban areas, thus making the sludge a non-attractive fertilizer source. In recent years, there has been concern about the sustainability of this state of affairs as regard to wastewater handling, as well as concern about the fate of the final waste deposits in the environment.

This concern was compounded during a period where farmers organizations advised their members to refuse receiving sludge before a guarantee was given by the Danish EPA that their food would not be considered unsafe. While such a guarantee was never given farmers actually refused to receive sludge for a prolonged period, leaving local municipalities with heaps of sludge, and stimulating a costly incineration of sludge. Organic farmers felt particularly strongly that something should be done about the state of affairs, and requested that some research into alternative waste management systems was carried out. Furthermore, since maintenance of the established systems will necessitate considerable investments in the future, there has been an interest in alternative systems for wastewater management.

An integrated study commissioned by the Danish EPA was undertaken to develop and evaluate technical solutions for increasing recirculation. It included assessment of technical solutions feasibility in a specific, medium-modern town setting – the municipality of Hillerød, and tentatively scaling up the implementation of such solutions to a national level. This work was based on selecting appropriate technologies for lessening some adverse ecological impacts of existing built environments as well as new urban structures to be built from the ground. Thus, in one sense it is on the edge of ecological engineering (Bergen et al., 2001), since it is partially limited by retro-fitting into existing structures. The main thrust of this assessment is the increasing use and recycling of food and energy resources, and thus a partial recovery of the ecosystems affected (back to food).

Section snippets

The resource production model

In order to assess the recirculation potential from Danish households, a resource production model was set up (Table 1).

Since persons spend only a part of their time in the household premises, it will be possible to recover only a fraction of the different wastes directly from the households. The recoverable fraction is assumed to be:

  • (1)

    50% of urine,

  • (2)

    75% of feces,

  • (3)

    90% of the solid organic waste,

  • (4)

    90% of the grey water (water used for washing, bathing and kitchen purposes).

On this basis it was

Results

The resource production overview demonstrates that the primary volume of waste is limited to approximately 800 L/person/yr, including physiological excreta, kitchen waste, and bathing and washing resources (mainly soap and detergents), see Table 1. However, when the water usage is included, the total volume is increased by a factor 70 to approximately 56 m3/person/yr.

It is notable that the urine fraction contains 50–67% of the total macronutrient resource thus exceeding the solid fractions (feces

Discussion

In the formulation of the project it was implicitly assumed that a recovery of nutrients and organic matter from the urban areas should be returned to the local food system. This assumption seemed obvious in the local Danish context but is in itself a value statement (sensu Bergen et al., 2001) that occurs naturally from the local context: Denmark is a small country that is strongly tied into the global fluxes of nutrients and energy, not least through its highly intensive animal husbandry

Conclusion

It is technically possible to design integrated ecological waste management systems, based on known components that may be operated at or close to the cost level of the current conventional sanitation systems. A number of possibilities exist for increasing recirculation of nutrients and organic matter from urban areas and reducing the energy consumption related to waste management. Such integrated systems need to be technologically developed and tested on a scale that will allow an interaction

Acknowledgements

We acknowledge contributions made by Technical staff from Hillerød municipality, and the Danish EPA. Furthermore we thankfully acknowledge the help that Dvoralai Wuhlfsohn gave during the revision of this paper.

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