Volatile acids concentration

Results for individual volatile acid concentrations in raw sewage determined by the direct injection procedure of Narkis and Henfield-Furie [578] and by that of Standard Methods [579], The results were also expressed as acetic acid for comparison with the collective total amount of organic acids determined by the Standard Method [579], The total amount of organic acids determined according to the Standard Method is higher than that found by the Narkis and Henfield-Furie [578] method. On average between 85 and 98% of the organic acids determined by the Standard Methods procedure were found to be volatile acids by the direct injection method. 

Volatile Acids Concentration. The rate of increase in volatile acids concentration, a dynamic variable, has been considered as the best individual indicator of the need for control action. Since the process is a series reaction, as shown in Figure 1, this increased rate can be caused... 

At this stage of development the model is restricted to a constant pH reactor and considers only two (pH and volatile acids concentration) of the five variables considered important for monitoring digester operation. This restriction of constant pH can be removed and the model extended to incorporate the interaction with bicarbonate alkalinity by considering the carbon dioxide-bicarbonate equilibrium as shown in Equations 16 and 17. 

The reaction term in this balance is zero since it has been assumed that neither C or A participate in other reactions such as precipitation or solubilization. With the bicarbonate concentration now known from Equation 20, the pH can be calculated from Equation 17 for constant values of dissolved carbon dioxide. The model is no longer restricted to a constant pH and now considers three (pH, volatile acids concentration, and bicarbonate alkalinity) of the five desired variables. However, it is restricted to a constant dissolved carbon dioxide concentration. 

Process failure by organic overloading was simulated by a sudden or step change in So from 167 to 667 mmoles/liter, and the results are shown in Figures llA and IIB. The volatile acids concentration and percentage of carbon dioxide in the gas phase both increase and the bicarbonate alkalinity and pH both decrease in a manner similar to that... 

Representative data from Run 1. which did not achieve steady state, as previously mentioned, are shown in Table V. This represents the baseline run without added nutrients. It was found that to maintain pH in the 6.8-7.2 range, 72 meq NaOH// of feed slurry was required this raised the sodium ion concentration in the digester to about 1670 mg//. Overall, the results of Run I were poor. The methane yield, volatile solids reduction, and energy recovery efficiency as methane in the product gas were low, and the volatile acids concentration in the digester effluent was high. 

The performance of Run 1 and the compositional data indicating possible nutritional deficiencies led to the evaluation of Runs 2, 3, and 4 at the same operating conditions as Run 1 except that the mixed nutrient solution in Table IV was added to the feed slurry to raise the concentrations of the nutrients. Sufficient nutrient formulation was added to reduce the C/N ratios of Runs 2, 3, and 4 to 16.3, 12.3, and 8.3, respectively. Substantial improvements were observed in the performance of these runs, but the volatile acids concentrations in the digester effluents were still high. Also, there did not seem to be a correlation between gas production and the concentration of added mixed nutrient solution. 

Run 7 exhibited the best methane yield of 3.51 SCF/lb VS added and the highest volatile solids reduction and energy recovery efficiencies of Runs 1 to 7. Also, the volatile acids concentration in the digester effluent is in the range... 

It should be pointed out that the performance of the packed-bed methane digester is more dependent perhaps on the influent volatile acids concentration than the overall VS loading rate. Thus, filter performance with acid-digester filtrate might be expected to be better than that obtained with methane-digester effluents having comparable volatile acids content. 

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