## 3 Results and Discussion
### Process Performance
These are substances that show toxicity only after exceeding a certain threshold value. The non-dissociated forms of these acids show toxic effects against methanogens. They can penetrate the cell membrane of microorganisms, where they dissociate when neutral, releasing a hydrogen ion that disrupts the acid-base balance in the reactor [35]. In almost all reactors, low VFA concentrations below 300 mg/L were observed, with the exception constituting the S2 supplied by OW. In this case, in the digestate an enhancement of its content by 40% was observed as compared to the control series (S1). An increase in the VFA concentration might have been the result of the presence of toxic substances in the feedstock, mainly d-limonene. Alkalinity informs about the buffer capacity of the digesters. It allows for a quick assessment of the stability of the process under the conditions of VFA accumulation. Values in the range of 2000 to 5000 gCaCO\({}_{3}\)/m\({}^{3}\) are considered favorable [36; 37]. As in the case of VFA, the deterioration of this parameter was found only in two-component co-digestion of OW and MSS. Importantly, the supplementation of feedstock in IPW ensured adequate buffering capacity in the digester (S4). As a result of the AD process, the release of ammonium nitrogen was observed. Major 3.1-fold growth was achieved in S1 and S2. Importantly, the highest concentration was obtained in two-component MSS and OW co-digestion. A different trend was noticed in the case of phosphates, wherein a reduction in this parameter was observed. In all co-digestion series, the removal efficiency was established at a similar level of 56%; in turn, in MSS mono-digestion, it reached a value of 36%. Another important parameter that was analyzed in this study was the COD/TN ratio, which is considered a key factor for a balanced performance of the AD process [38]. It should be pointed out that the introduction of co-substrates resulted in an improvement in this ratio, with the greatest in S4 supplied by both ICW and OW, wherein the COD/TN ratio was established at the level of 29.7, while in the control it was 26.1. In both co-digestion series (S2 and S3), comparable results were achieved, reaching a value of 28.4. Additionally, in this research, the variation in nutrients during AD was investigated. In the presence of co-substrates, increased contents of both nitrogen and total phosphorus were observed. Major enhancements were achieved in three-component series supplied by both OW and IPW. Regarding TN, in almost all series (S1-S3) an enhancement in its content was achieved during AD. The exception was S4, wherein comparable concentrations in feedstock and digestates occurred. In the case of TP, an analogous tendency was observed, e.g., in S1-S3 there was an improvement in concentration during AD. In turn, comparable concentrations in feedstock and digestates were observed in S4. Considering the possibility of further use of digestates for fertilizing, it is worth mentioning that the highest contents of both TN and TP were achieved in S3 supplied by IPW. Previous studies indicated that digestate from co-digestion of fat-oil-grease (FOG) and sewage sludge has better fertilizing properties due to an additional nutrient content provided by those wastes.