Assessment of anaerobic domestic wastewater treatment: case study of Ciudad de la Costa WWTP in Uruguay

Abstract

In Uruguay, sanitation and water supply provision is responsibility of the governmental company OSE (Spanish acronym for State waterworks), as well as the operation of WWTPs. Most of the plants use biological treatment to remove organic matter and nutrients, as activated sludge, aerobic ponds for small villages, and anaerobic treatment. The Ciudad de la Costa WWTP was designed for biological removal of organic matter using up-flow anaerobic sludge blanket (UASB) and chlorination as post-treatment to remove the pathogens, with a maximum capacity of 344.000 P.E. Nowadays, this unique plant using anaerobic treatment for domestic wastewaters, and is in operation since July of 2015. The plant that was being operated by the contractor, and in July of 2016, passes into the hands of the state company OSE. Due to this, most of the problems of the plant, are related to the protocols of operation, and the learning of the personal to handle it. Hence, the aim of this work is evaluate the performance of the plant in this stage. The average inflow of this period is 3780 m3/d, receiving wastewater from Pando, and Ciudad de la Costa city. This last one is conveyed by trucks, because the sewer system it is being constructed. The average composition of the influent is 899 mgCOD/L, and 361 mgBOD/L, and the efficiency in terms of removal of BOD is 75%, and TSS is 147 mg/L. Also, the average temperature of the influent is 20oC, reaching a minimum value of 9oC. The loading rate is high compared with the low SMA reported in January of 2016, but this value depends of the quality of the sludge, and varies with the time, so is recommended to do another analysis. Besides of this, the alkalinity indexes were analysed, and are according to a stable process. For determine the current situation, two sampling campaigns for the physiochemical parameters, were necessary to determine the characteristic of the wastewater, including sulphates, sulphides and iron. The objective is to analyse the impact of iron chloride, dosed at the pumping station, in the anaerobic reactors. As well as, the relation with the production of H2S in the reactors, that can lead to the inhibition of methanogenic activity. From the data measured, the sulphate is in the range of 30 to 40 mgSO4/L, and the total iron from 2 to 11 mgFe/L. However, the relation between COD/SO4 is high so the inhibition of methanogenics is not expected. Regarding the mass balance of sulphates, the efficiency in the removal is in average 43%. The sulphate removed is reduced to H2S, it is dissolved in the effluent, and released in the discharge structures, due the high turbulence generated. As well as, is accumulated in the sludge as FeS. Due the lack of information related to the biogas production, this value is estimated from the COD balance. The methane yield estimated is between 0.10 - 0.17 Nm3CH4/kgCODremoved, an average production of 500Nm3/d of biogas, assuming 70% of methane.Finally, a model is implemented using a PetWin™, simulator from EnviroSim Ltda., developed for industrial effluents, which includes the sulphate reduction in the process. Based in the data of November, the model is implemented, and calibrated. But to have an accurate model, measurements of the biogas production are required. The parameters more sensitive were the methanogens yields, while the yields of the SRB don’t affect the model, due the low concentration, all the sulphate was reduced into H2S, the results shows values less than 1% of H2S in the biogas composition. Although, the validation of the model is not achieved with the data of February, the model fits with the average values of the plant. Due to this, for an accurate model, information of biogas production and composition is required.