What is wastewater treatment capacity and how is capacity selected?

There are many factors considered during the design of a wastewater treatment plant (WWTP).  Design is primarily based on what capacity the wastewater system should be designed to treat.  Capacity can be broken down into two categories – hydraulic capacity and biological capacity.   

Hydraulic capacity is the ability of the treatment systems to maintain or pass a given liquid flow rate through each of the treatment processes. Biological capacity is the ability of the treatment systems to handle and treat the organic loads delivered from the collection system. Most of the organic load comes from domestic wastewater which primarily comes from homes.   Biological treatment relies on bacteria and other microorganisms to break down organic wastes.

Throughout the design process, population trends are reviewed along with projected areas of development to establish a baseline hydraulic flow and projected biological and nutrient loadings to the WWTP.  This projection typically covers a 20-year planning cycle to coincide with the useful life of many pieces of equipment and infrastructure.  

Capacity, both in the form of hydraulic and biological, greatly impacts the capital costs to expand the plant’s treatment capabilities and impacts the long-term operating costs of the facility.  For this reason, it is important to right-size the improvements to offer a cost-effective solution.  Providing excessive capacity that may never be utilized is cost prohibitive to the community and increases operating costs over the useful life of the project.  For this reason, it is important to select design capacities based on reasonable growth projections for the selected planning period.

During planning stages, the City and its engineering consultant reviewed historical flow and loading data to develop the project scope for the current project.  During that review, it was determined that the City’s WWTP was operating at its design biological capacity and improvements were needed to expand the facility’s ability to treat incoming wastewater.   The proposed project aims to address the City’s projected capacity requirements for the next 20 years.

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1. What is wastewater treatment capacity and how is capacity selected?
2. Why is the City’s WWTP located near the Grand River?