Kirchmann Holger, Börjesson Gunnar, Kätterer Thomas, Cohen Yariv
Department of Soil and Environment, Swedish University of Agricultural Sciences, P.O. Box 7014, 75007, Uppsala, Sweden.
Ambio. 2017 Mar;46(2):143-154. doi: 10.1007/s13280-016-0816-3. Epub 2016 Sep 20.
The composition of municipal wastewater and sewage sludge reflects the use and proliferation of elements and contaminants within society. In Sweden, official statistics show that concentrations of toxic metals in municipal sewage sludge have steadily decreased, by up to 90 %, since the 1970s, due to environmental programmes and statutory limits on metals in sludge and soil. Results from long-term field experiments show that reduced metal pollution during repeated sewage sludge application has reversed negative trends in soil biology. Despite this Swedish success story, organic waste recycling from Swedish towns and cities to arable land is still limited to only about 20 % of the total amount produced. Resistance among industries and consumers to products grown on land treated with sewage sludge may not always be scientifically grounded; however, there are rational obstacles to application of sewage sludge to land based on its inherent properties rather than its content of pollutants. We argue that application of urban organic wastes to soil is an efficient form of recycling for small municipalities, but that organic waste treatment from large cities requires other solutions. The large volumes of sewage sludge collected in towns and cities are not equitably distributed back to arable land because of the following: (i) The high water and low nutrient content in sewage sludge make long-distance transportation too expensive; and (ii) the low plant availability of nutrients in sewage sludge results in small yield increases even after many years of repeated sludge addition. Therefore, nutrient extraction from urban wastes instead of direct organic waste recycling is a possible way forward. The trend for increased combustion of urban wastes will make ash a key waste type in future. Combustion not only concentrates the nutrients in the ash but also leads to metal enrichment; hence, direct application of the ash to land is most often not possible. However, inorganic fertiliser (e.g. mono-ammonium phosphate fertiliser, MAP) can be produced from metal-contaminated sewage sludge ash in a process whereby the metals are removed. We argue that the view on organic waste recycling needs to be diversified in order to improve the urban-rural nutrient cycle, since only recycling urban organic wastes directly is not a viable option to close the urban-rural nutrient cycle. Recovery and recycling of nutrients from organic wastes are a possible solution. When organic waste recycling is complemented by nutrient extraction, some nutrient loops within society can be closed, enabling more sustainable agricultural production in future.
城市污水和污泥的成分反映了社会中元素和污染物的使用及扩散情况。在瑞典，官方统计数据表明，自20世纪70年代以来，由于环境项目以及对污泥和土壤中金属的法定限制，城市污泥中有毒金属的浓度已稳步下降，降幅高达90%。长期田间试验结果显示，在反复施用污泥期间，金属污染的减少扭转了土壤生物学的负面趋势。尽管瑞典有这样的成功案例，但瑞典城镇向耕地的有机废物回收利用仍仅占总产量的约20%。行业和消费者对在施用污泥的土地上种植的产品的抵制，可能并不总是有科学依据的；然而，基于污泥的固有特性而非其污染物含量，将污泥施用于土地存在合理障碍。我们认为，将城市有机废物施用于土壤对小型市政当局来说是一种有效的回收利用形式，但大城市的有机废物处理需要其他解决方案。城镇收集的大量污泥没有公平地返还到耕地，原因如下：（i）污泥中的高含水量和低养分含量使得长途运输成本过高；（ii）污泥中养分对植物的有效性低，即使经过多年反复添加污泥，产量增加也很小。因此，从城市废物中提取养分而非直接进行有机废物回收利用是一条可行的途径。城市废物燃烧增加的趋势将使灰烬成为未来的一种关键废物类型。燃烧不仅会使养分在灰烬中富集，还会导致金属富集；因此，通常不可能将灰烬直接施用于土地。然而，可以通过去除金属的过程，用受金属污染的污泥灰生产无机肥料（如磷酸一铵肥料，MAP）。我们认为，为了改善城乡养分循环，对有机废物回收利用的观点需要多样化，因为仅直接回收城市有机废物并非实现城乡养分循环闭合的可行选择。从有机废物中回收和再利用养分是一种可能的解决方案。当有机废物回收与养分提取相结合时，社会中的一些养分循环可以闭合，从而在未来实现更可持续的农业生产。
