Sewage sludge as a fertilizer

Osady ściekowe

Closed-loop economy (GOZ) is an extremely important concept to make us use available natural resources more efficiently. In some sectors, GOZ is more obvious and has been known for years – selling second-hand clothing or things in antique stores. In others, the best solutions are still being sought. For the municipal sector, an opportunity to operate in a closed loop is to work with agriculture. What is waste (sewage sludge) for the treatment plant, for farmers can be a valuable source of nitrogen and phosphorus compounds, or simply fertilizer.

Sewage sludge in agriculture – requirements

Sewage sludge can be used agriculturally. However, given that they are derived from wastewater treatment and may therefore be contaminated, a number of requirements have been introduced for their use. The first, and very important, is that only sludge from municipal wastewater treatment plants can be used as fertilizer. This excludes the use of highly contaminated sludge of industrial origin.

They must also be thoroughly tested for heavy metals and parasites. Importantly, sewage sludge cannot be applied to plants for direct consumption.

Sewage sludge as an element of fertilizers

Another way to use sewage sludge agriculturally is to add it to fertilizers and soil conditioners. The Ministry of Agriculture has long observed an increasing number of applications for permits to market fertilizers and crop support products made from waste, including sewage sludge.

Advantages and disadvantages of using sewage sludge

The use of sewage sludge in agriculture has both good and bad points. We asked the Ministry of Agriculture about the pluses and dangers associated with them. The answer given emphasized that the use of sewage sludge as raw materials for the production of fertilizers and plant aids (mainly for improving soil properties) is part of the so-called “soil improvement”. closed-loop economy.

Sewage sludge, used as a raw material for the production of fertilizers and plant growing aids, once the appropriate permit is issued by the Ministry of Agriculture to market it, in accordance with the requirements of the Law on Fertilizers and Fertilization in conjunction with Art. 14 of the Waste Law, they lose their waste status and function on the market as a full-value product (fertilizer or soil conditioner), closing the recycling process. In addition, fertilizers and soil conditioners made with sewage sludge complement the mineral fertilizer market. They are also very competitively priced.

Detailed rules for the proper use of fertilizers containing nitrogen in their composition, which include sewage sludge, are defined primarily by the Water Law Act and the Decree of the Council of Ministers on the program of activities aimed at reducing water pollution by nitrates from agricultural sources and preventing further pollution, the so-called nitrate program. It specifies all aspects related to the agricultural use of municipal sewage sludge within the framework of current law. The specific conditions that must be met for the agricultural use of municipal sludge are specified in a regulation of the Minister of the Environment.

Nitrogen from sewage sludge should be taken into account as part of balanced fertilization with maximum amounts of working nitrogen. The nitrate program, updated in 2023, was supplemented with a method for calculating the amount of nitrogen in sewage sludge using the results of the study, and Table 11 was supplemented with fertilizer equivalent values for sewage sludge by spring and autumn application date (0.3 and 0.25, respectively).

Regarding the problems that exist with the agricultural use of fertilizers and soil conditioners that have been produced using sewage sludge, the first thing to point out is:

  • improper behavior of producers of this type of fertilizer: changes in raw material composition, compared to that approved at the stage of issuing a permit by the Ministry of Agriculture. An arbitrary change in raw material composition can make a fertilizer an environmentally hazardous product;
  • contaminants that do not comply with the standards prescribed for fertilizers and crop support products. In the event that the Ministry of Agriculture obtains information about violations from inspection authorities, it will initiate an investigation, which may end in the revocation of the permit.

How many sewage sludge products have been registered?

According to the Ministry of Agriculture, in 2022. 220 permits were issued for the marketing of fertilizers and crop support products, including 30 for products made from stabilized municipal sewage sludge. 2022 also issued:

  • one decision revoking a permit to market an organic-mineral soil conditioner made from stabilized municipal sewage sludge;
  • one decision upholding a decision revoking a permit to market fertilizer, produced from stabilized municipal sewage sludge;
  • one decision refusing to issue a permit to market an organic soil conditioner made from stabilized municipal sewage sludge.

Large-scale treatment plants vs. agricultural use of fertilizers

The larger the wastewater treatment plant, the more sludge it produces. That’s why we asked facilities next to major urban centers whether they give up sludge for agricultural use. Gdansk Waterworks, MWPiK in Warsaw and Krakow City Waterworks responded that their sewage sludge is not used in agriculture. It remains an open question whether they are managed in other, more profitable ways, such as for biogas production, or whether they do not meet statutory standards.

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