Cost confrontation study for decentralized wastewater treatment: When to adopt a cluster or onsite system?

In Brazil and in many other developing countries, the collection and treatment of domestic sewage is not universal. A solution to this problem would be the implementation of decentralized systems. However, there is a lack of studies that can contribute to the decision-making process on the best system to be adopted: onsite or cluster. Thus, this study evaluated the implementation costs of sewer main line and wastewater treatment system, aiming to determine when the onsite system (septic tank + seepage pit) is more economically viable than the cluster system (septic tank + anaerobic filter). The design calculation for each technology was conducted based on the Brazilian's standards and the number of inhabitants was variable between 4 people for one single residence, up to 1,000 inhabitants for cluster system. Then, an analysis evaluated the system's cost for implementation. The results pointed out three distinct scenarios, showing the interference of distance between households in the total cost of the system. In distances up to 18 m, the cluster system is economically more viable than the onsite system; between 19 and 75 m, the cluster system is more viable than the onsite from a minimum number of contributors; from 76 m on, the onsite system will always be the most economically viable.