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Vurdering af muligheder og begrænsninger for
recirkulering af næringsstoffer fra by til land 

Abstract

In this project 14 handling systems for domestic wastewater and organic kitchen waste has been described and evaluated. A method for choosing systems for different housing areas in a city has been developed. The method has been used for Hillerød city, where four handling systems where chosen. There are several advantages by using the four systems instead of the already existing sewer system. The energy surplus is the same as the energy consumption for 900 households, the nutrient collected with in the system is enough to fertilise 451 hectare of agricultural field. Even though the yearly costs are only 17 percent higher than in the existing system.

Closing the loop.

In the beginning of the last century, night soil from the cities where sold to the farmers. The agriculture in the peri urban areas where often based on this kind of fertiliser. The entry of the water closet and the sewers broke the circulation of nutrient from the cities to the agricultural field. To prevent unhygienic conditions on the bathing beach the waste water pipe was therefore extended. In the 1960`s it emerged that the discharges was harmful for the marine environment and wastewater treatment plants were build. However the wastewater treatment resulted in a huge sludge production. The increased requirements for lower contents of environmental hazard compounds in sludge have caused that less sludge can be used as fertiliser.
The sewer system and the wastewater treatment plant today manage all wastewater treatment. The purpose of this project was to set up and evaluate handling systems that could contribute to return nutrients to agricultural areas in order to close the nutrient cycle between urban and rural areas. The evaluated technologies were primarily technologies, which collects the toilet and kitchen waste separately to avoid pollution with environmental hazard compounds. The collecting of human urine has been in focus in previous projects because it is rich in nutrients and is not complicated to use because of hygienic restrictions. All the wastewater types from the household are described in this report as well the handling of the organic kitchen waste. This has been done since it should be possible to compare the described system with the existing wastewater handling system. The described systems has not been chosen according to whether it is legal or not to use the treated waste products in agriculture

Description of the handling systems.

There were chosen well-known technologies in the description of the 14 systems in the report. Allthough the composition of the different technologies were not tested for all the systems. The technologies were chosen from a prerequisite that the participators in the project were willing to live with the systems. Eight of the 14 systems are connected to the sewers while the rest of the systems are working with out connection to the sewer. The systems that are connected to the sewer system are constructed in preparation for implementing in existing cities. The remaining six systems which are more space demanding are described in preparation for establishing of new buildings. The systems are quantitatively descriebed concerning economy, energy consumption and the amount of nutrients, which are collected and can be recycled. Eight other parameters are described qualitativly among others reliability, maintenance and sturdiness. In a multi-criteria analyses the 14 systems are evaluated in proportion to each other and to six housing types. For the six housing types there has been made recommendations of systems in the light of outdoor area and the evaluation. The project describes a method to choose systems for townships in a city, which makes the results useful for other cities than the case city in this report. Below the method is used on the case city Hillerød.

Selected systems for Hillerød

When choosing a system for a city there is a necessarity to consider the local conditions. Before choosing the systems the housing types must be characterised, the amount of waste estimated and the housing type situation must be mapped. The evaluation of the systems and the housing types in the city will define the kind of systems there will be chosen. The method used on Hillerød City where the number of inhabitants is 26.818 persons. Before choosing the system Hillerød was divided in to nine housing areas, with different kind of housing. For each area there were chosen a handling system, which were considered the best regarding the housing type. Four handling systems were chosen for the nine areas. In Hillerød system one (se figure 1) is chosen for housing in the centre of the town. The system was chosen because there where not space enough for implementing systems with separate urine or faeces collection. It would also be difficult to collect urine with a truck in the narrow streets. System number 2 was chosen for housing areas with self-contained houses, row houses, apartments, and for house in the rim of the centre. In these areas there were enough space for collecting tanks and for local use of organic kitchen waste. Even though the kitchen waste was used for a biogas plant because there is a big potential of energy in the waste. It was decided not to collect faeces because of the many apartment houses in the area. In our scenaria collecting of faeces implies use of vacuum toilets, and the noise from these toilets would be annoying in apartment houses. System number 3 are chosen for housing areas with row houses and self-contained houses. Collecting of faeces is possible here because it is houses with only one family. System number 4 are chosen for allotments, because they are not provided with sewers. The allotments are the only kind of housing where it is possible to use all the waste products locally because the ground here is big enough. A general conclusion is that the closer to the town centre the fever systems can be used because there are more limited space for the collecting technologies.

Figure 1 Look here!

Consequence of the system change

The 4 chosen systems for Hillerød are compared with a reference system. As reference system are chosen a system where the organic kitchen waste are composted in a central place while the waste water are cleaned in a conventional waste water treatment plant. The reference system represents a system, which are used in many cities today. The chosen systems are compared with the reference system according to economy, energy consumption and the amount of nutrients collected. Comparing the energy consumption in the four chosen systems with the reference system (table 1) shows that there is an energy gain by choosing the four systems and an energy consumption by choosing the reference system. System number 4, chosen for allotments and system number 1 chosen for the town centre are a bit cheaper than the reference system, while system 2 and 3 are a little more expensive. All the chosen systems are more effective regarding recycling of nitrogen. In recycling potassium and phosphor there is no big difference.

Table 1
Comparison of energy consumption, economy and recycling potential person /year. For the four chosen systems and the reference system. A negative energy consumption are an energy surplus.

Table 2
Comparison of the reference system with the four chosen systems for 26.818 persons in Hillerød city

As described in the introduction the existing wastewater treatment system is formed by a row of historical incidences The sewer system and later on the wastewater treatment plants has been extended gradually as the problems occurred. The investments that have been done in the existing system must be depreciated but when renovating and building new houses it is important to investigate alternative options. In the long run the new handling systems are more sustainable than the existing technology because they meet the demand that the societies are putting forward. Using the method described in the project report it is possible to chose systems that will increase the recycling of organic waste. There will not only be chosen one system, because the method consider the housing type, but a number of handling systems that can work in parallel or together.

Other results

In relation to the project there were collected urine that was investigated for microbiological parameters, nutrients and environmental hazard compounds. The urine was collected from urine source separating toilets from 4 different housing types. The examination showed that use of human urine at least could be used under the same conditions as sludge from wastewater.