By Chris Strycharz, PE


A significant portion of wastewater is generated by facilities in rural and remote areas. These facilities typically do not have access to a municipal sewer hookup, which presents the design engineer with a unique set of challenges and obstacles when designing an onsite wastewater treatment plant. How much flow will be generated? What are the wastewater characteristics of this facilities? Is there enough land to fit a system? The key to solving these challenges is to perform an accurate site analysis. 

Prior to beginning the process design, the engineer must first understand the input and output parameters for the treatment plant. These parameters include flow rate, influent wastewater characteristics, effluent treatment requirements, and site conditions. The major challenge faced with onsite wastewater treatment plant design is the variation between projects. The facility type and how it is operated will dictate the flow generation and wastewater characteristics. 

The four basic steps discussed below will help the design engineer gather the required information so that they can begin the treatment plant design process. Other considerations beyond these four will depend on the specific system, regulations, and treatment needs:

1. Determine Flow Rate

The first step in the design process is to determine the Average Daily Flow rate (ADF). For new construction, flow rate tables located in local regulatory standards can be used to determine the total ADF for the project. For existing or replacement systems, water meters located on the facility or even flow meters on the existing treatment plant may be used to quantify the project’s ADF. The flow volume will directly impact the size of the system and the available treatment types that can handle this flow. High flow systems tend to have more consistent wastewater characteristics, while low flow systems tend to be more susceptible to fluctuation in wastewater strengths. This fluctuation may lead to a strain on the treatment plant if not properly addressed. Flow pattern, such as seasonal or intermittent use, is another important aspect that can be considered in the design process. The maximum daily flow rate should be considered based on the facilities type and a peaking factor may be used to determine the maximum daily flow rate. Projects with peak flows may utilize flow equalization to buffer spikes in the flow to not overload the treatment system. Sites with seasonal use, like campgrounds, may consider designing parallel treatment trains that can be turned on/off when the facility requires additional or reduced capacity. The same design may be used for projects with a phased construction schedule. 

2. Determine Influent Wastewater Characteristics

The next step in the design process is to determine the influent wastewater characteristics that will be used in the design of the treatment plant. As noted previously, facilities generate varying strengths and characteristics of wastewater. For example, a convenience store will have much higher strength wastewater compared to an apartment complex due to the type of wastewater generated. The main wastewater constituents used in the design process are BOD (Biochemical Oxygen Demand), TSS (Total Suspended Solids), TKN (Total Kjeldahl Nitrogen), TP (Total Phosphorus), and FOG (Fats, Oils, and Grease). Accurately determining the wastewater strengths will allow for a properly sized system. For existing or replacement systems, sampling and testing may be used to determine wastewater characteristics. New construction requires a clear understanding of projected facility operations and what will be going down the drain. Literature and other technical resources are available that offer typical wastewater strengths based on different facility types. Wastewater treatment plants are not designed on flow alone, flow and load need to be a basis for design. The load is calculated from the average daily flow rate and the concentration of the wastewater constituent being analyzed.

3. Determine Effluent Treatment Criteria

After the influent wastewater has been characterized, the next step is to determine the required treatment parameters of the effluent that exits the treatment plant. The treatment criteria will help guide the design engineer to the correct process that can achieve these treatment levels. In many cases, the discharge method will determine what effluent targets are required by the local regulations. Three common discharge methods are point source discharge, soil dispersal, and reuse. The local regulatory authority will determine the treatment levels and sampling frequency based on the effluent discharge method associated with the treatment plant. It is common for the design engineer to assess the benefits and drawbacks of each discharge method with the cost, size, and complexity of the treatment plant that is required to achieve the different treatment standards. Typically, soil-based dispersal requires less stringent treatment standards compared to reuse or point source discharge. This may reduce the cost, size, and complexity of the treatment plant if soil-based dispersal is specified.

4. Understand Site Conditions

The final step in the site analysis is to understand the specific site conditions. Common site conditions under consideration in the design process are site layout, soil conditions, elevation, temperature, power availability, noise, and odor. 

  • Site layout – Site layout will help the design engineer determine the treatment and discharge options available. Sites with limited space for soil dispersal may be forced to discharge to a point source which may lead to tighter effluent requirements. 
  • Soil conditions – Soil type, percolation rate, and depth to restrictive layers, help the engineer determine the options for soil dispersal or even soil treatment. 
  • Elevation – Elevation is a parameter that is used in the aeration design of an aerobic treatment system. Higher elevations have lower oxygen concentrations compared to sea level which requires additional air to be introduced to the system to meet the oxygen requirement for the system. 
  • Temperature Wastewater temperature is factored into the design, especially in colder climates. Colder temperatures lead to slower biological reaction rates, which may lead to increased treatment system size. 
  • Power Availability – The voltage, phase, and frequency available at the site is required to select the equipment, which includes pumps, blowers, mixers, etc. 
  • Noise – Noise may be a concern with larger treatment plant equipment like blowers. Understanding the facility type and potential noise concerns will help the engineer design a system that reduces these issues. Site placement of the treatment plant and equipment away from people is a simple solution. Sound attenuating structures or structures that redirect noise are commonly used as well. Another option is the use of silencers on blowers. 
  • Odor – Like noise, odor control should be a step in the design process to ensure that odors are appropriately handled. Carbon vent filters or mulch beds are two examples of cost-effective solutions to reduce odor concerns. Since every onsite project is different, the site conditions will change and must be addressed on a case-by-case basis.


The challenge presented with onsite wastewater treatment plant design is the variation between projects. Following the four basic steps listed above is a useful tool for design engineers to gather the crucial site-specific information for their project. The key to a properly designed treatment plant is to understand and determine the input and output parameters. Before the engineer can jump into the process design, they must first perform a thorough site analysis. Flow rate determination is the first step that will ensure an appropriately sized system. Accurate design values for influent characteristics will ensure the treatment plant is sized appropriately. Assessing the different discharge methods will lead to the effluent treatment criteria of the project, which then drives the treatment process selection. Finally, an understanding of the unique site conditions will improve the overall design and efficiency of the system. After these steps are completed, the design engineer should have sufficient information for the accurate design of the treatment plant. This upfront analysis will lead to an efficiently designed treatment plant that is catered to meet the needs of the project.  

Chris Strycharz, P.E. joined Infiltrator Water Technologies in 2015 after graduating from the University of Connecticut with a BS in Civil and Environmental Engineering. Starting his career as a Project Engineer, Chris worked in the Technical Services Department and contributed to the R&D of new products. In 2019, Chris took on a new role as the Western Region Engineered Systems Consultant, where he was responsible for providing sales and technical assistance for commercial and decentralized onsite wastewater systems. As of 2022, Chris began a new role as a Wastewater Engineer assisting in the design of onsite wastewater treatment facilities. Chris is a registered PE in the State of Connecticut and recently received his Master of Engineering degree from the University of Connecticut.