## 4 Discussion
This effectivity was primarily assessed in terms of %OM and COD decreases and, in general, both parameters behaved similarly. Overall, the BOD\({}_{5}\) values showed the same pattern except for the treatment containing acetate, where it also decreased but to a lesser extent compared to COD. This yielded a higher BOD\({}_{5}\)/COD relationship and suggests that, in this treatment, there was still enough biodegradable material to further increase the reduction of OM. However, it should be noted that, in anaerobic environments, BOD\({}_{5}\) could not be a totally appropriate parameter to show the real biodegradability as in sediments OM degradation should be more related to metabolisms that do not use oxygen as an electron acceptor. On the other hand, the highest decrease in TP content was also shown by the treatment including BAP amended with acetate, which together with the increase in loosely-P, may show the release of inorganic-P from the organic phosphorus as organic matter is degraded. In previous studies using Acti-zymeTM in wastewater treatment plants [10; 12], the concentrations employed were usually lower (only occasionally around 50 g/m\({}^{3}\)) than the standard dose used in our experiments. Concentrations assayed here were selected based on earlier studies carried out in rivers also located in the Jucar River basin [5]. Our findings indicate possible shortcomings of using lower amounts (i.e., 25 g/m\({}^{3}\)), or of diluting excessively the product by a high spatial spreading, in such a way that the threshold concentration for the effective functioning of the BAP in natural environments should not be much lower than that used here as standard (i.e., 50 g/m\({}^{3}\)). This could be the case, at least, for river sediments, which have renewal times much longer than those of the sewage systems. For treating sewage, however, using lower amounts of BAP than those assayed here, but with sustained maintenance amendments, would be enough. Although our study was not clearly able to solve if the performance of the BAP could be optimized by modifying some basic aspects of the application protocol, a certain improvement in the degradation of organic matter was found by increasing the dose of BAP. However, differences did not commonly reach the statistical significance with the standard BAP treatment. Only for the removal of N and P, some statistical significance was obtained that could motivate further exploration of this possibility. Differently to the assessments conducted for the application of BAP (i.e., changes in the amount, temporal dosage, spatial dosage), the gradual effects of the other treatments were not explored in this study. This is because they had an exploratory purpose to test its suitability for accompanying the standard BAP application. Accordingly, these compounds were provided at non-limiting concentration, in such way that its optimal utilization could be appraised in future experiments once they have proven its effectiveness, or neglected when ineffective. An important outcome of our study is that acetate clearly benefits the performance of the BAP in reducing the organic and nutrient content. This could have different causes, which might all lead to the production of methane. Acetoclastic methanogens, sulfate-reducing and denitrifying bacteria, all share acetate as electron donor [16; 38], and all them were present to some extent in the studied sediments as demonstrated by the NG-TAS analysis. On the other hand, acetate is also used by the syntrophic acetate oxidizing bacteria [37; 39], which in turn produce H\({}_{2}\) or formate that can later be used by both hydrogenotrophic methanogens and sulfate-reducers. An experiment conducted with different inocula from biogas reactors showed that additions of acetate caused a shift from a predominant pathway of biogas generation by syntrophic acetate oxidizing bacteria and hydrogenotrophic methanogens, to another consisting in the direct utilization of acetate by acetoclastic methanogens [40].