C+S February 2022 Vol. 8 Issue 2 (web)

Introduction High strength waste (HSW) presents a challenge for any type of onsite wastewater treatment system. This is apparent in restaurant facilities, where the design life of the system is significantly less than a typical residential system. It can also be a factor in other types of businesses that generate HSW. Each facility type will have unique wastewater characteristics as well as unique site and soil conditions for the system designer to consider. The more information the designer can obtain in advance via effluent sampling, water meter records, usage patterns about the distinct, and potentially problematic conditions, the better the HSW has been defined by many agencies and publications and varies accordingly. The State of Georgia regulations define HSW as greater than 200 mg/l Biochemical Oxygen Demand (BOD) or Total Sus- pended Solids (TSS). However, there may be constituents other than BOD/TSS that would make the waste stream “high strength” including but not limited to pH (too high or too low), fats, oils & grease (FOG), or nitrogen (N). HSW has been loosely defined within the industry as anything greater than residential waste strength. HSW and Code Minimums Even among similar establishments, HSW values vary greatly, and many health codes fail to address that inconsistency or are too broad to provide cohesive guidance. Effective HSW best practices may need to exceed local code minimum requirements in order to be effective and some of them discussed here fall into that category. The system designer should review the costs and benefits of any additional, recom- mended design features with the system owner. Ultimately, the decision to increase the reliability and longevity of the wastewater system at the outset and incur potentially increased costs resides with the owner. Safety Factors to Consider proposed design. What is HSW? Sizing of onsite wastewater systems for single-family homes is typi- cally based on the estimated peak daily flow and the Long-Term Ac- ceptance Rate (LTAR) of the soil for residential strength septic tank effluent. In most states and provinces, the design flow is based on the number of bedrooms in the house and a daily flow of 150 gallons is commonly assumed for each bedroom. This daily flow per bedroom as- sumes two people per bedroom who generate 75 gallons per day (gpd) each. Bedrooms, rather than current occupancy, are used for the basis of design because the number of occupants in the house can change. Decentralized High Strength Waste Facility Design Recommendations and Best Practices By Dennis F. Hallahan, P.E.

Using this typical estimating procedure, a three-bedroom home would have a design flow of 150 gpd/bedroom x 3 bedrooms or 450 gpd. However, the actual daily average flow could be much less. Based on the 1990 census, the average home is occupied by 2.8 persons. Each person in the United States generates 45 to 70 gpd of domestic waste- water. Assuming these averages, the average daily flow would be 125 to 195 gpd or 28 to 44 percent of the design flow, respectively. The higher design flows compared to the actual flows result is longer reten- tion times within the septic tank, possibly 6-7 days, which provides protection to the drainfield. These factors of safety are pointed out for residential systems, however most of these are neglected in the design of commercial systems there - by resulting in a much shorter design life. The flows may be accounted for however the HSWmay not be accounted for, resulting in residential strength LTAR’s to be incorporated with HSW loading. HSW Best Practices: Design for Code Conformance: At a minimum, the design should conform with state-provincial/county rules and regulations. Please note that codes are a minimum design threshold, and the design can incorporate additional features, superseding the code required mini- mums. For example, depending upon facility type, it may be good practice to specify additional septic tank capacity, advanced treatment, larger drainfields, or alternating/resting drainfields, etc. Research Wastewater Characteristics and Facility Operations: Conduct research to understand the facility type, the wastewater characteristics, and operations within the facility. There is much data available online. If it is an existing facility, then visit the facility. Ob- serve practices including cleaning habits and products, and disposal of those products. Quaternary ammonia products are great sanitizers however can wreak havoc on treatment systems. Consult with main - tenance providers and obtain historical records. This research only takes a small amount of time, but it may yield valuable information on unique issues that should be accommodated in the design or ad- dressed otherwise. Pressure dosing with the use of flow equalization can help with the performance of HSW systems.



February 2022

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