By Dennis F. Hallahan, P.E.
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 proposed design.
What is HSW?
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 Suspended 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, recommended 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
Sizing of onsite wastewater systems for single-family homes is typically based on the estimated peak daily flow and the Long-Term Acceptance 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 assumes 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. 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 wastewater. 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 retention 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 thereby resulting in a much shorter design life. The flows may be accounted for however the HSW may 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 minimums. 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. Observe 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 maintenance 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 addressed otherwise.
Obtain Sample Data: Sampling data could be collected at an existing facility or, if the facility is new, sampling data can be obtained from similar usage facilities. Some states may have requirements for the number of samples and the sample collection times. Consult with the local permitting authorities about proper procedures. Typical parameters for sampling include BOD, TSS, FOG, N(series), pH, and temperature.
Consider Increased Tank Volume: Greater retention times can reduce wastewater strength. In many areas of the US and Canada, it is common practice to double the size of the code-required grease trap. Grease types have changed, and dishwasher temperatures have increased, yet codes have not kept pace with the changes. For septic tanks, a minimum 48-hour retention time is recommended, although some codes only require a 24-hour retention time.
Assess Soil Loading Rates: Please note that the soil loading rates in codes are typically based upon residential strength waste. Soil loading rates can be reduced to account for HSW. Spreading out the effluent over a larger footprint will provide better long-term performance.
Design Based Upon Mass/Organic Loading: Determine the pounds of BOD per day for the system. Use this information along with organic soil loading rates to determine the size of the system. (The State of North Carolina is a good example that can be used as a reference.)
Incorporate Pressure Dosing/Time Dosing: Pressure and time dosing can spread out the BOD load over a greater area.
Integrate Flow Equalization: For peak flow event facilities, such as a church, party/wedding venue, stadium, or weekend restaurant, designing the system with increased pump tank storage to accommodate flow equalization would allow the dose to be spread out over an extended amount of time. Flow equalization is a great choice for advanced treatment systems because it will give a consistent flow and waste load to the treatment device, which provides better operating conditions.
Consider Pretreatment Options: Providing advanced treatment can reduce the strength of the effluent. Options include Advanced Treatment Units (ATUs) and numerous ATU types are available. While code approvals of treatment technologies differ, any code approved ATU is acceptable. Consult the ATU manufacturer on ATU size and specifications based upon waste strength and flow.
Simple aeration devices such aeration only or MBBR-type systems offer a low-cost option to reduce wastewater strength. Verify which products are approved in the state/province in which you are working. These devices can pretreat the effluent down to residential levels, thereby allowing the use of residential loading rates. This will protect the drainfield and increase the system lifespan.
Employ Outlet Filters: Septic tank outlet filters can provide a performance enhancement. However, they require routine maintenance more often than the maintenance to the tank itself. Therefore, design with maintenance in mind with access risers. Review the owner’s current maintenance contract (if they have one) to include filter maintenance. Contact the outlet filter manufacturer to specify the correct filter size and type. If possible, oversize the filter to protect the system during abusive or high flow events and/or install multiple filters. Filters can be installed in parallel configuration and some manufacturers offer an alarm feature. Some wastewater systems utilize venting and rely on air flow through the septic tank and in this case, an outlet filter may not allow the drainfield to vent back through the system as it should. Therefore, when an outlet filter is specified or required, designers can also specify plumbing that still allows airflow through the systems.
Plan Operations and Maintenance: O&M is the Achilles Heel of onsite wastewater industry. The value of a good O&M plan and the quality of the provider cannot be overstated. The service provider can monitor the system, check performance, adjust the operating system to specifically meet the facility operations, and notice potential problems before they become major problems.
There is no one solution that will fit all cases. Each site and facility is different. For example, If an RV park with HSW is only in use for 5-7 months per year, it may be good practice to increase tank sizes and include flow equalization to handle fluctuations and peak flows. This allows the drainfield to remain at the minimum code required size with the understanding that the system can rest and recover for many months during the off season. Drainfield monitoring can also be incorporated as part of the O&M plan to safeguard the system.
The decentralized wastewater industry has come a long way and the future looks bright. Various treatment technologies to address High Strength Waste are available that were once only available for large scale systems, and designers and installers have become more educated. Rules and regulations have improved to account for HSW, and the all-important O&M void is being filled by qualified professionals. The design decisions are ultimately the responsibility of the system designer and the owner. All designs and solutions will vary. The appropriate solution may incorporate a combination of these recommendations and there may be additional best practices not covered here. All designs are required to meet the minimum code requirements and should follow engineering best practices. The goal is to provide a system that will perform well for the customer, meet the regulatory treatment levels, and protect public health and the environment.
Dennis F. Hallahan, PE has more than 30 years of experience with onsite wastewater treatment systems’ design and construction. Currently Technical Director at Infiltrator Water Technologies, he is responsible for technology transfer between Infiltrator and the regulatory and design communities and consults on product research and testing for universities and private consultants. Dennis received his MS in civil engineering from the University of Connecticut and his BS in civil engineering from the University of Vermont. He is a registered professional engineer in Connecticut and holds several patents for on-site wastewater products.