Decentralized Wastewater Treatment Approaches Meet Community Treatment and Large Commercial
System Demands By Dennis F. Hallahan, PE and Donald Prince
Introduction Two general methods are considered for wastewater treatment plan- ning and design: centralized and decentralized models. As environ- mental and community challenges become more complex and budgets for infrastructure projects continue to be diminished, large commercial projects and communities that do not have access to a centralized wastewater treatment plant or where the plant is at capacity are utiliz- ing decentralized approach to wastewater treatment. This approach can serve the community and sustain development without huge infrastruc- ture costs. The growing awareness of nutrient damage to the environment from nitrogen and phosphorus is the latest in a push for increasingly stringent health codes to protect vulnerable environments. These codes, enacted by states and municipalities, are challenging engineers and develop- ers to propose wastewater treatment solutions that can perform long term and protect the environment without a huge community finan- cial burden. The value of preserving the water resource is recognized worldwide as one of the greatest challenges of our time. Background Centralized and decentralized wastewater treatment systems vary greatly as does the impact they have on environmental health and po- table water supplies. Centralized systems obtain potable water from one location; that water is utilized by the public then collected as wastewater and transported to the treatment plant following which it is discharged to surface waters. This process short-circuits or bypasses the local water cycle and can cause aquifer depletion. This can be par- ticularly challenging in coastal areas where salt water intrusion is a concern. The decentralized model collects, treats, and then discharges to the subsurface all within a local area. This has the benefit of replacing the original water resource back to the local aquifer at a much lower energy cost. When adequately designed, installed, and maintained decentral- ized wastewater systems have the capacity to process large quantities of wastewater into the underlying soils, making this option one of the most passive and sustainable forms of aquifer recharge. Rather than partially to fully treating wastewater effluent then discharging it to a surface pond or injection well before recharging the aquifer, decentral- ized systems can provide both wastewater treatment and groundwater
AES pipe connection to the tanks in Tuscan Ridge.
recharge in one step. Via this sustainable practice, the replenished aqui- fer can then supply wells, recharge wetlands for wildlife, maintain base flow, and in the case of coastal cities and towns, counteract saltwater intrusion. Decentralized Approaches for Large Commercial and Com- munity Applications While decentralized systems continue to serve the rural areas outside city limits, the notion that the decentralized system is only there to serve small, single family homes has been transformed with large decentralized systems handling flow rates more than 1 MGD. Large businesses and communities no longer need wait or pay exorbitant tap fees to tie-in to existing centralized services. Often these large com- mercial systems utilize a combination of technologies that could be traditionally considered on one or the other side of the centralized/ decentralized fence. Cluster systems In many communities, where centralized wastewater treatment facili- ties (WWTF) are overburdened and the addition of new sewer lines is prohibited, or where individual septic systems are frowned upon, cluster systems are being recommended to developers by local health departments and planning agencies.
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csengineermag.com april 2020
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