Development of a Method to Evaluate Decentralized Cluster Wastewater Systems as an Alternative for Cesspool Replacement

Abstract

Throughout Hawai’i there are approximately 88,000 cesspools that release an estimated 53 million gallons per day (mgd) of wastewater into the environment and pose an environmental and public health risk. ACT 125 was passed by the state legislature in 2017 to ban all cesspools in the state by the year 2050. It is expected that cesspools will be replaced by onsite wastewater treatment (OSWT) and disposal systems located on individual properties or connection to sewers and offsite wastewater treatment facilities. In some cases, however, where several cesspools are in proximity, it may be feasible to construct small-scale, decentralized cluster wastewater systems for several homes on a neighborhood level. These systems will require wastewater collection, treatment, and disposal. This report provides a statewide study of potential neighborhoods/sites for these small-scale/cluster systems with an initial focus on priority areas, including planning level cost estimates. The method found in this report uses GIS and density-based analysis across Oahu, Maui, the Big Island, and Kauai to determine the overall feasibility of cluster systems in Hawaii, evaluate specific clusters in priority areas, and make cost comparisons between OSWT’s and cluster systems. The results indicate that the replacement of clusters of cesspools in Hawai’i with decentralized wastewater systems is feasible. While cluster systems are more expensive than onsite systems regardless of the size of the cluster or the site conditions, the difference is smaller in areas that require nitrogen removal from OSWT’s and in areas that contain more houses/cesspools. Lastly this study leads to the following recommendations for future work: 1) evaluate large clusters of cesspools for possible centralized or heavy-duty decentralized treatment systems; and 2) design a collection, treatment, and disposal system for a specific neighborhood.