C+S November 2022 Vol. 8 Issue 11

Four Basic Steps to Begin an Accurate Site Analysis for an Onsite Wastewater Treatment Plant Design

By Chris Strycharz, PE

Introduction 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 waste - water 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 under - stand the input and output parameters for the treatment plant. These parameters include flow rate, influent wastewater characteristics, efflu - ent 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 de - pend 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 lo - cal 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 waste - water 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 com- plex due to the type of wastewater generated. The main wastewater constituents used in the design process are BOD (Biochemical Oxy - gen 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 prop- erly sized system. For existing or replacement systems, sampling and testing may be used to determine wastewater characteristics. New con - struction 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


November 2022 csengineermag.com

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