The following parameters were determined: total and soluble COD, pH, total solids, mineral solids, volatile solids, total suspended solids, mineral suspended solids, volatile suspended solids, total volatile fatty acids (TVFA), alkalinity and total phenolic compounds. All analyses were carried out according to the recommendations of the Standard Methods of American Public Health Association (APHA, 1989).

In each steady-state experiment, samples were collected and the above parameters analysed. The pH and gas volume were determined daily, whilst the remaining parameters were measured at least five times per week on five different samples taken on different days to ensure that representative data were obtained.

2. Results and discussion

2.1 Influence of substrate concentration and OLR on the COD removal efficiency and operational parameters

The anaerobic degradability studies were carried out using two different two-phase OMSWs with COD concentrations of 35 g COD/L (OMSW 1) and 150 g COD/L (OMSW 2). The experiments were performed using progressive influent substrate concentrations, those corresponding to the OMSW 1 being the first ones and those corresponding to the OMSW 2 carried out at the end of the study.

Tables 2 and 3 summarize the steady-state operating results including HRT, OLR, methane production rates (rcm), total and soluble CODs, VS, TVFA, alkalinity and TVFA/alkalinity ratio for the OMSW 1 and OMSW 2, respectively (Borja et al., 2002).

OLR (g COD/(L d))

Fig. 1. Variation of the percentage of COD removed with the OLR for the two OMSWs used (■: OMSW 1; •: OMSW 2).

Values are the averages of 5 determinations taken over 5 days after the steady-state conditions had been reached. The differences between the observed values were less than 3 % in all cases. (* rCH4: methane production rates)

Table 2. Steady-state results under different experimental conditions for the OMSW 1 with a COD of 35 g/L.

Table 3. Steady-state results under different experimental conditions for the OMSW 2 with a COD of 150 g/L.

As can be seen in Figure 1 the percentage of COD removed decreased with increased OLR for the two influent substrate concentrations studied. The percentage of COD removal decreased from 93.3% to 83.2% when OLR increased from 0.86 to 4.14 g COD/(L d) for the most diluted substrate (OMSW 1). For the most concentrated influent (OMSW 2) OLRs were varied from 3.00 to 15.03 g COD/ (L d) and COD removal efficiencies higher than 88% were obtained at an OLR of 12.02 g COD/ (L d). Even under a higher OLR of 15.03 g COD/(L d), corresponding to an HRT of 10 days, COD removal was 82.9%.

The total effluent CODs of the anaerobic reactor increased with increased OLR for the two influent substrate concentrations studied, as summarized in Tables 2 and 3. Such an increase in the effluent COD was paralleled by a similar increase in the effluent total volatile fatty acids (TVFA). This seems to indicate that, at higher OLR, the effluent total COD and mainly soluble COD is largely composed of the unused volatile acids produced in the reactor.

Given that the buffering capacity of the experimental system was found to be at favourable levels with excessive total alkalinity present at virtually all loadings, the efficiency of the process and the rate of methanogenesis was not very affected. The experimental data obtained in this work indicate that a total alkalinity of about 1.7 g/L as CaCO3 is sufficient to prevent the pH from dropping to below 7.0 at an OLR of 9.05 g COD/(L d) for the most concentrated substrate used (OMSW 2).

The pH in the reactor was always higher that 7.0 for all the HRTs and OLRs studied corresponding to the most diluted OMSW studied. In addition, pH values equal or higher than 6.9 were observed for OLRs lower than 12.02 g COD/(L d) and HRTs higher than 12.5 d when the most concentrated influent was processed, with pH of 7.2 as a maximum value achieved. This high stability can be attributed to carbonate/bicarbonate buffering. This is produced by the generation of CO2 in the digestion process which is not completely removed from the reactor as gas. Buffering in anaerobic digestion is normally due to bicarbonate, as carbonate is, generally, negligible if compared to the bicarbonate (carbonate/bicarbonate ratio is equal to 0.01 for pH 8.2) (Speece, 1983). The buffering guards against possible acidification of the reactor giving a pH of the same order as the optimal for methanogenic bacteria (Wheatley, 1990).

The TVFA/ Alkalinity ratio can be used as a measure of process stability (Wheatley, 1990): when this ratio is less than 0.3-0.4 the process is considered to be operating favourably without acidification risk. As was observed in Tables 2 and 3 the ratio values were lower than the suggested limit value for OLRs lower than 9.05 g COD/ (L d) in the experiments corresponding to the highest influent substrate concentrations studied (OMSW 2). For this substrate, between HRTs of 50.0 and 16.6 days, the TVFA/Alkalinity ratio was always lower than the above-mentioned failure limit and the TVFA values were always lower than 1,08 g/L (as acetic acid). However, at a HRT of 10.0 days, a considerable increase of the TVFA/ Alkalinity ratio until a value of 0.95 was observed in the reactor, which was mainly due to a considerable increase in the TVFA concentration (1.57 g/L as acetic acid) with simultaneous decrease in alkalinity (1.32 g/L, as CaCO3).