Obtain Sample Data: Sampling data could be collected at an exist- ing 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. Typi- cal 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 or- ganic 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 dos- ing 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, design- ing 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 treat- ment 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 Treat- ment 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 specifica- tions 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 work- ing. 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 perfor- mance 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. Conclusion 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 incor- porated 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 educat - ed. 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 design- er 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 waste- water 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.
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February 2022 csengineermag.com
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