Abstract
The complexity of the operation of the anaerobic digestion turns important to use a stability test to verify if the operating conditions are under control and the process is working as required. The current method to verify the process stability by the VFA/TA ratio consists of bands, but the stability bands vary according to the organic material. Indicators such as pH or constant biogas production are also used. Therefore, the aim of this study was to propose the use of Shewhart control chart for individual measures such as anaerobic digestion stability test and to demonstrate its use with the anaerobic digestion of poultry litter. This method showed to be advantageous because it standardizes a variation range for the process according to its average and verifies the stability by direct measures, such as the efficiency of organic material removal and biogas production.
Highlights
IntroductionThe methods that have been used to evaluate anaerobic digestion (AD) stability are indicators as pH, volatile fatty acidity and total alkalinity (VFA/ TA) ratio and constant biogas production
The methods that have been used to evaluate anaerobic digestion (AD) stability are indicators as pH, volatile fatty acidity and total alkalinity (VFA/ TA) ratio and constant biogas production.The pH measure is considered less important than the VFA/ TA ratio for representing large changes in alkalinity, with small variations on a logarithmic scale
The AD was evaluated by the organic material removal efficiency, which was measured by the amount of total volatile solids consumed (TVScons, %) and by the biogas production, being expressed by the ratio between these variables (m3biogas kg-1 TVScons-1)
Summary
The methods that have been used to evaluate AD stability are indicators as pH, VFA/ TA ratio and constant biogas production. The pH measure is considered less important than the VFA/ TA ratio for representing large changes in alkalinity, with small variations on a logarithmic scale. Experimental results have shown that, in the event of a drastic pH drop due to VFA accumulation, AD process rarely returns to normal operation (MÉNDEZ-ACOSTA et al, 2010). Sought attributes for system monitoring have to be low cost, low maintenance, easy to use, and produce fast and accurate responses (JOBLING PURSER et al, 2014). In this context, a statistical process control could be used as stability test for the AD process
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