Abstract

Recycling of plant nutrients in sewage products to arable land is regarded as a step towards a more sustainable society, as today’s use of scarce resources is a threat to future generations. Sewage sludge is the predominant sewage fertiliser product available today. However, the use of sewage sludge in agriculture is a controversial issue. Blackwater from separating systems seems to fulfil agricultural requirements on a fertiliser product to a higher degree than sewage sludge without many of the hazardous substances in sludge. The main objective of this study was to analyse environmental impact and resource use in systems integrating wastewater management and agriculture. The methodology used was based on LCA methodology. Three different wastewater systems were evaluated together with the agricultural production potentially affected. The first alternative (the Surahammar reference system) represented a system that a large part of the population in Surahammar is connected to today. Food waste disposers were installed in 50% of the households, wastewater was treated in a conventional wastewater treatment plant and the sewage sludge was used for production of a soil conditioner. The second alternative (the sludge utilisation system) symbolised a wastewater system more commonly represented in Swedish municipalities, and included also agricultural use of the sewage sludge. The third alternative (the blackwater system) was based on a future vision of Surahammar. Here, toilet water from low-flush vacuum toilets and milled organic waste were digested separately. The blackwater product was stored and spread in growing oats, thereby replacing mineral fertilisers. Data on the wastewater system were derived from simulations by the substance-flow model URWARE/ORWARE. Assumed conditions for the agricultural system were mainly based on site-specific data from a farm producing according to the Svenskt Sigill label. All energy used and all emissions were related to a functional unit. The functional unit included wastewater treatment for 8830 persons in Surahammar, disposal of the sewage products and production of 2100 tonnes of oats on 486 hectares. As regards the operating phase of the whole system, the blackwater system required less fossil fuel and electricity than the other two systems. Although the blackwater system required considerably more fossil fuel for collection and transport, the total use of fossil fuel was lower due to a reduced need for mineral fertiliser. Construction of storage tanks and pipes for the blackwater system may, however, give a considerable contribution to the total environmental load. Including the construction phase too revealed that the use of fossil fuel increased considerably and was highest from the blackwater system. The emissions of greenhouse gases were of the same magnitude for all three systems, although slightly lower for the blackwater system. This was also evident when the construction phase was considered. The eutrophying emissions were reduced considerably in the blackwater system according to a maximum scenario, mainly due to reduced emissions of ammonium from the treatment plant. The emissions of SO2 were of the same magnitude for all three systems studied, although slightly lower for the sludge utilisation system, As regards NH3 and NOx, the emissions were highest for the blackwater system. A high substitution of mineral fertiliser, an optimal spreading technique and an appropriate design of the system for collecting and storing the blackwater were highlighted as important for many environmental aspects in the blackwater system.

Keywords

blackwater; environmental systems analysis; source separation

Published in

Rapport. Miljö, teknik och lantbruk
2005,
Publisher: Institutionen för biometri och teknik, Sveriges lantbruksuniversitet

Permanent link to this page (URI)

https://res.slu.se/id/publ/125991