ABSTRACT The anaerobic digestion (AD) operation complexity raise the importance of stability testing to verify whether the operating conditions are under control and whether the process is working as required. The current method of verifying process stability is by a range of the ratio between volatile fatty acidity and total alkalinity (VFA/ TA) within which stable conditions are brought about; these rates vary with the content of organic material. A few indicators 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 as a stability test for AD, as well as to demonstrate its use in poultry litter AD. The method showed to be advantageous since it standardizes a variation range for the process according to the average and verifies the stability by direct measures, such as organic material removal efficiency and biogas production.

As fecularias produzem grande volume de manipueira que, estabilizado em lagoas, provoca desperdício da energia do biogás e a poluição da atmosfera. Para contribuir com a reversão dessa realidade, desenvolveu-se esta pesquisa de tratamento de manipueira de fecularia em reator anaeróbio tubular horizontal piloto, com meio suporte, em peças de bambu. O reator foi escavado no solo e impermeabilizado com geomembrana, com volume de 33,6 m³ e alimentação contínua por gravidade. Os indicadores de estabilidade foram pH, relação acidez volátil/alcalinidade total e produção de biogás. O delineamento experimental foi o inteiramente casualizado, com respostas submetidas à análise multivariada. As cargas orgânicas em termos de demanda química de oxigênio (DQO) foram de 0,556; 0,670; 0,678 e 0,770 g L-1 e em sólidos voláteis (SV) de 0,659; 0,608; 0,570 e 0,761 g L-1 para os tempos de retenção hidráulica (TRH) de 13,0; 11,5; 10,0 e 7,0 dias, respectivamente. As reduções médias de DQO foram de 88; 80; 88 e 67% e de SV de 76; 77; 65 e 61%, respectivamente. As produções de biogás por DQO consumida foram de 0,368; 0,795; 0,891 e 0,907 L g-1, por SV consumidos de 0,524; 0,930; 1,757 e 0,952 L g-1 e volumétrica de 0,131; 0,330; 0,430 e 0,374 L L-1 d-1. O reator permaneceu estável, e as peças de bambu, em análise visual ao final do experimento, apresentaram-se fisicamente íntegras.

The cassava starch industries generate a large volume of wastewater effluent that, stabilized in ponds, wastes its biogas energy and pollutes the atmosphere. To contribute with the reversion of this reality, this manipueira treatment research was developed in one phase anaerobic horizontal pilot reactor with support medium in bamboo pieces. The reactor was excavated into the ground and sealed with geomembrane in HDPE, having a volume equal to 33.6 m³ and continuous feeding by gravity. The stability indicators were pH, volatile acidity/total alkalinity ratio and biogas production. The statistical analyses were performed by a completely randomized design, with answers submitted to multivariate analysis. The organical loads in COD were 0.556; 0.670; 0.678 and 0.770 g L-1 and in volatile solids (VS) of 0.659; 0.608; 0.570 and 0.761 g L-1 for the hydraulic retention times (HRT) of 13.0; 11.5; 10.0 and 7.0 days, respectively. The reductions in COD were 88; 80; 88 and 67% and for VS of 76; 77; 65 and 61%. The biogas productions relatively to the consumed COD were 0.368; 0.795; 0.891 and 0.907 Lg-1, for the consumed VS of 0.524; 0.930; 1.757 and 0.952 Lg-1 and volumetric of 0.131; 0.330; 0.430 and 0.374 L L-1 d-1. The reactor remained stable and the bamboo pieces, in visual examination at the end of the experiment, showed to be in good physical conditions.