This is the first study that identifies nitrogen loss and drier liquor parameters in a full scale sludge drying facility. Obtained data enabled proposing novel treatment based on deammonification process that allows not only nitrogen removal from drier liquor but also from gravity thickeners supernatant, the stream that is considered too cold for deammonification. The novel treatment approach is compared with nitrification/denitrification in activated sludge process in terms of greenhouse gas emissions. The nitrogen loss during drying was calculated based on two independent methods using full scale data from a long-term measurement campaign. According to results, 9.8-11.2% of nitrogen from dewatered sludge, or 4.9-5.5 g N/kg of TS, was vaporized during drying and then was captured through the purification of process air in drier liquor. Overall, over 40 tN/a from drier liquor and 77 tN/a from gravity thickeners supernatant is recirculated to the activated sludge process and results in the additional emission of 670 426 kg CO_2e/a when nitrification/denitrification is applied. That can be lowered by 398 858 kgCO_2e/a when the novel approach is considered.

The deficiency of readily biodegradable organic carbon can be a significant limitation to effective nitrogen removal during wastewater denitrification. Waste-activated sludge (WAS) is a source of carbon produced directly at wastewater treatment plants (WWTPs). Raw WAS has a large molecular weight and complex chemical structure molecules that are not easily available for microorganisms. In this study, easily biodegradable organic fractions were released using pH control and/or nitrites and nitric acid (NO₂/FNA). The obtained results indicated that WAS can be a sufficient carbon source for denitrification in WWTPs that are at risk of minor effluent violations. The implementation of WAS disintegration with the use of pH control and NO₂/FNA allowed for the denitrification of an additional 0.5 and 0.8 mgN-NO₃/L. WAS disintegration, besides being a source of carbon generation, reduces the volume of sludge and leads to the implementation of a closed-loop system.

The nitritation process (ammonia oxidation to nitrite) is successfully applied in different processes used for reject water treatment. The impact of pH was investigated in experiments performed at pH 6.0, 6.5, and 7.0 using real dewatering liquor. The overall process performance in terms of the effluent quality and stability was compared. Efficient nitratation suppression was achieved at pH 6.5 and below due to the strong free nitrous acid (FNA) inhibition of nitrite oxidizers. The nxr gene activity showed the highest NOB suppression at pH 6.5. The process rate depended on the pH and nitrogen loading, and was in the range of 13.2–16.5, 8.7–23.3, and 34.0–37.9 g N/(kg·VSS·h), at pH 6.0, 6.5, and 7.0, respectively. The highest inhibition constant was estimated to be 0.905 g HNO2-N/m3 at pH 6.0. Decrease in pH also improved the AOB tolerance to free ammonia up to Ki, NH3 = 97.2 mgNH3-N/dm3. The affinity constants were 0.40, 0.34, and 0.12 mg NH3-N/dm3 at pH 6.0, 6.5, and 7.0, respectively. Microbial analysis revealed that the flocculation could be attributed to the Accumulibacter and Competibacter extracellular polymeric substance (EPS) production, which can also serve as a protector agent for nitrifiers.