Compost and sewage sludge

A yellow wheel loader implements composting in a composting plant.Click to enlarge
Wheel loader being used to turn compost
Source: Tim Hermann / UBA

Compost and sewage sludge contain valuable nutrients and humus substances that farmers traditionally use as organic fertilizer so as to reduce mineral fertilizer use. But unfortunately, compost and in particular sewage sludge, contain pollutants such as chemicals, heavy metals and pharmaceutical residues that are hazardous to the environment and human health.

Introduction

Composted organic waste from households, businesses, yards and parks, food processing and agricultural waste, as well as sewage sludge from municipal sewage treatment plants contains a host of valuable plant nutrients. Rich in nitrogen, phosphorous, potassium and trace elements such as copper and zinc, such compost is used as an organic fertilizer by farmers. Apart from providing plant nutrition, the organic substances contained in compost stabilize and/or improve soil humus content. Compost can also be used as a substitute for horticultural peat, as well as for mineral fertilizers, whose production process is energy intensive This also helps to reduce greenhouse gases. Using phosphorous recovered from wastewater and waste is also beneficial in light of the finite nature of this resource.

Environmental impact

But apart from nutrients, compost and above all sewage sludge also contain the following inorganic pollutants: toxic heavy metals such as lead, mercury, cadmium, copper and uranium; organic toxins such as dioxin, PCB , PFT, pharmaceutical residues, pathogens and so on. When compost is used as fertilizer, the pollutants in the compost can accumulate in the soil and enter the food chain via crop plants. This can be hazardous to groundwater and surface waters if the pollutants end up in such waterbodies as the result of run-off or percolation. The environmental and health hazards posed by compost fertilizer are difficult to determine because the interactions and transformation processes of compost pollutants are often unknown.

Fertilizer legislation

Against this backdrop and in the interest of minimizing environmental hazards, only approved fertilizers may be used. The fertilizer regulation stipulates that organic-residue fertilizer such as compost and sewage sludge must be labelled as such and may only be placed on the market if their proper use is hazardous to neither soil fertility, human, animal and plant health, nor ecosystems. To this end, the regulation lays down minimum requirements concerning the following: labelling; adherence to pollutant limit values; epidemic control; plant health. However, as many pollutants are unregulated and the hazards posed by them are indeterminable, some regional states have banned the use of sewage sludge as fertilizer. Maximum allowable pollutant and heavy metal content is based on the precautionary principle, pursuant to the soil protection regulation. The organic waste and sewage sludge regulations stipulate detailed restrictions, bans and regulations concerning the use of sewage sludge and organic waste as agricultural and horticultural fertilizers, and set differing pollutant limit values for clayey, loamy and sandy soils.

The amount of sewage sludge and organic waste that can be used as fertilizers is subject to statutory limits. For example, up to five tons of dried sewage sludge may be used for each hectare of land over a given three year period, and the concurrent use of fertilizer containing organic waste is prohibited. In the interest of preventing pathogen transfer, the use of sewage sludge for organic farming, permanent pastures, forests, and produce crops is prohibited. Apart from the legal regulations concerning the use of organic-residue fertilizers, growers are also required to adhere to the code of good agricultural practice under the fertilizer regulation, as is the case with all other fertilizers.