Technological parameters of compost production based on sewage sludge and cereal straw

One of the main issues of modern agricultural production in the conditions of the climate crisis and growing moisture deficit is to preserve soil fertility. This issue is especially acute in Ukraine, where manure production per hectare of arable land has fallen by more than 10 times in recent decades due to declining livestock numbers. One of the few sources of organic raw materials that can be used to maintain soil fertility is municipal sewage sludge. According to generalized data, the nitrogen content in sewage sludge is 1.6–7% in dry matter, reaching 7.4% in activated sludge, the ratio of nitrogen to carbon varies from 1:11 to 1:5, and sludge moisture depending on the processing technology is in the range from 72 to 93%. It is established that for effective composting of sewage sludge it is necessary to add to the compost mixture carbon raw materials, the most expedient type of which is the non-grain part of the harvest, straw. It is determined that the rational ratio of dry matter of sewage sludge and cereal straw in a balanced compost mixture will be, respectively, 29% and 71%. This ratio also reduces the relative content of inorganic pollutants (plastic, heavy metals, etc.) contained in the sludge by 2.76 times. As a result of the numerical experiment, an adequate regression model of the influence of the factors of sewage sludge moisture and the required nitrogen-carbon ratio on the moisture content of the compost mixture was obtained. Using the obtained model, it was found that for the maximum recommended values of compost mixture moisture (65%) and nitrogen-carbon ratio (C:N = 30), the maximum moisture content of sewage sludge that can be used to balance the compost mixture without additional moisture is 88.7%, which is lower than the values of sludge moisture after settling tanks. With this in mind, advanced technological schemes for wastewater treatment with compost production were proposed.