Controlling low-temperature drying facilities which utilise nonprepared air is quite difficult, due to very large variability of ventilation air parameters - both in daily and seasonal cycles. The paper defines the concept of cumulative drying potential of ventilation air and presents experimental evidence that there is a relation between this parameter and condition of the dried matter (sewage sludge). Knowledge on current dry mass content in the dried matter (sewage sludge) provides new possibilities for controlling such systems. Experimental data analysed in the paper was collected in early 2012 during operation of a test solar drying facility in a sewage treatment plant in Błonie near Warsaw, Poland.

The objective of this study is to recover sludge from the Dar Gueddari sewage treatment plant and use it in the cultivation of corn in the open field on clay soil. To do this, four doses were tested against two types of controls: a control without addition of sludge and another with the addition of nitrogen-based fertilisers (200 kg N, 100 kg P and 100 kg K). The short-term effects indicated that the application of the sludge had a beneficial effect on the fertilising qualities of the soil and therefore on the crop yield. The monitoring of the agronomic parameters of the maize plant showed that the growths and the yields varied according to doses of the sludge. In grain yield, the difference between plot E4 and E0 was around 3.3 Mg∙ha ^–1. In addition, the plots treated with large sludge doses experienced improvements in total nitrogen and organic matter. During the second season, the results obtained showed decrease in nutrients (nitrogen and phosphorus). It emerged from the C:N ratio which was 8. This indicated a good mineralisation of the organic matter (OM) which was average of 2.48 ±0.04% and 2.5 ±0.01%, respectively in the E4 and E5 treatments. In addition, the amounts of N, P and Ca in dry matter (DM) increased with increasing the dose of sludge. During the 2017 season, the highest values were detected in plot E5 with averages of 1.6% in N and 0.53% in P.