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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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Collection and treatment systems are typically designed to reduce the need for pumping wastewater and utilize gravity flow to the extent possible. Treatment facilities are normally located near rivers or streams for two reasons: low-lying areas allow the collection system to utilize gravity to deliver wastewater to the treatment facility, and it locates the facility to discharge the treated effluent to the receiving body of water.
The City’s WWTP was originally located and built in the 1950’s in its current location on the Grand River. The facility has undergone subsequent improvements projects in 1971 and 2012 to expand the facility and its treatment capabilities. Building a new WWTP in a different location would be prohibitively expensive for the City and would increase long-term operating costs of the system as wastewater would have to be pumped continuously versus using gravity flow. The more cost-effective solution is to upgrade the existing facility while implementing measures to address discharge during high river stages.
As part of the planning process for the current project, the City and its engineering team reviewed the impacts of sustained high river levels (such as during the 2019 ice jam) on the WWTP’s ability to continuously treat the wastewater and process the influent flows received throughout the collection system.
The proposed improvements include repairs in the collection system to reduce inflow and infiltration (I/I) that must ultimately be treated at the City’s WWTP. I/I can be defined as clean water that enters the collection system through cracks or seepage in the pipes from storm runoff, groundwater infiltration, or inflow from high water events. Decreasing the amount of I/I from the collection system to the WWTP frees up available hydraulic capacity for wastewater that needs to be treated. In addition, the proposed project includes hydraulic improvements and implementing provisions to enable the City’s operations staff to pump treated effluent to the river during high river stages.
In order to perform the necessary improvements to the WWTP, the City will be required to bond out for this Project. The anticipated cost is approximately $10 million with the City qualifying for 1.5 million in principal forgiveness though EGLE’s Clean Water State Revolving Fund (CWSRF). In order to support the bond and pay for the Project, rates will increase 29% beginning July 1, 2022 and are anticipated to increase another 25% for the fiscal year beginning July 1, 2023.