Volume 42 / Issue 2 March / April 2019

One Water Strategy WRF resources. Page 6

Project Management Artificial intelligence. Page 10

Water Reuse Economic benefits. Page 23

Membranes CCRO breakthrough. Page 26

Energy Recovery More biogas, higher revenues Decentralized Treatment Systems 39 Myth busting: The value of an interceptor tank in effluent sewer systems

An effluent sewer interceptor tank serves a valuable role in the entire collection and treatment system. Mike Saunders of Orenco Systems explains how it can generate significant savings by providing low-cost primary treatment.

Interceptor tanks are used in at any given time are in the range tees, bottomless tanks, collapsed reliable strength and structural effluent sewer systems to retain of 10-30 percent. As a result, septic or cracked tanks, and open tanks. integrity they provide. For this a large percentage of solids at tanks have somewhat of a bad Various studies that quantify septic reason, just like any wastewater the source of the wastewater, reputation. system failures typically show failed collection system that is properly with treatment occurring at the In order to better understand tanks as the primary issue in a designed, contracted, and individual sites. The treated – and septic systems and their alternatives, failed septic system, accounting for maintained, interceptor tanks used essentially solids-free effluent – is the following is an overview of the around 50 percent of total failures in effluent sewers have little in then conveyed for final treatment differences between a (Day, 2008). Failures in septic common with the aging septic tanks and disposal. Solids separation is and an effluent sewer interceptor tanks are primarily related to poor often found in failed septic systems. facilitated by an interceptor tank, tank, the important function an design, poor construction, poor An interceptor tank, as part of which is designed and operated interceptor tank performs in an maintenance, and – most notably – an effluent sewer, provides low- similarly to a septic tank. For this effluent sewer, and how to quantify aging beyond their useful life. cost, low-energy primary treatment reason, effluent sewer systems are the value of the interceptor tank An effluent sewer interceptor of wastewater. The constituents also referred to as Septic Tank through a very simple method. tank, when properly designed and of full-strength wastewater can Effluent Pump (STEP) systems A typical septic system consists constructed, is typically larger be in the range of 450 milligrams Unfortunately, sub-standard of a septic tank and some type than a septic tank and is often per liter (mg/L) 5-day biochemical septic tanks have negatively affected of subsurface treatment and constructed of fiberglass reinforced oxygen demand (BOD5), 503 mg/L the reputation of STEP systems disposal. Most septic systems rely polyester (FRP) or high-density (TSS), and – and, in turn, have affected the on homeowners for maintenance, polyethylene (HDPE). Strict 164 mg/L fats, oils, and grease general perception of interceptor with periodic inspection by local fabrication, testing, and installation (FOG). By comparison, typical tanks. For this reason, it’s important health officials. Problems that protocols are established and wastewater exiting an interceptor to understand the role of the occur in a septic system can include followed to ensure that the tank is tank following primary treatment is interceptor tank not only as part failure of the subsurface system, watertight and structurally sound. very low in FOG and is in the range of the collection portion but also piping failures, inadequate sizing, FRP and HDPE products are of 140 mg/L BOD5 and 30 mg/L as part of the entire wastewater inadequate maintenance, and failure commonly used in many waste- TSS. Based on these concentrations, collection and treatment system. of the septic tank itself. Tank issues water collection and treatment a residential connection serviced The good news is that the include missing baffles or outlet technologies due to the long-term, by effluent sewer will produce retention of solids in the interceptor tank generates significant cost savings that are often not quantified The good news is or considered when designing an integrated wastewater collection that the retention and treatment system. of solids in the Septic tanks have been in use for more than 150 years. Today, interceptor tank they service more than 21 million generates homes in the United States (US), or 20 percent of homes in the significant cost nation, as part of septic systems, savings that are according to the US Environment Protection Agency. Throughout often not quantified the US, existing septic systems are or considered aging, and a large percentage of these are near or past the end of when designing their useful life. For example, the Florida Department of Health an integrated estimates that there are 2.6 million wastewater septic systems in the state, of which 2 million are stated to be older collection and than 20 years. While there is no treatment system. Watertight interceptor tanks are an important component of reliable data available, conservative effluent sewers. They provide low-cost, low-energy, primary estimates of septic system failures treatment of wastewater. Photo by Orenco

World Water March / April 2019 Decentralized Treatment Systems 41

A Septic Tank Effluent Pump, or STEP, system should not be perceived negatively based on the reputation of sub-standard septic tanks used in an onsite septic system.

Left: Interceptor tanks are unobtrusive and can easily blend into landscaping, as evident at this residence in Vero Beach, Florida, United States. Photo by Orenco approximately 189 fewer pounds use, less biosolids handling, and less headworks and sludge disposal. still measurable. A watertight, of BOD and 286 fewer pounds of biosolids disposal. These combined TSS has a major impact on costs high-quality interceptor tank, TSS annually. expenses can make up a substantial associated with solids handling, while similar in concept to a septic In an effluent sewer, the majority portion of the total wastewater clarifiers, and solids disposal. tank, is not merely a septic tank. of the solids are broken down and treatment cost. Rather than detail Actual costs are highly variable, It’s a valuable component of a digested as part of primary treat- the operational savings in various depending on the type of treatment wastewater collection and treatment ment, while the solids that cannot processes, this analysis uses the process and operating protocols. process, generating significant off- be broken down anaerobically surcharges implemented by actual Based on review of actual surcharge setting savings through the low- accumulate in the interceptor tank. utilities for BOD and TSS. These derivations, variable costs that can cost primary treatment that occurs According to Orenco Systems, these surcharges are typically assessed be impacted by BOD and TSS range within the tank. solids can typically be retained for per pound of treated BOD and TSS from 15-40 percent of the total A Septic Tank Effluent Pump, more than 10 years, depending on to industrial users that produce surcharge. For a single residential or STEP, system should not be per- the maintenance protocols being high-strength waste. In practice, connection, $20-50 annually ceived negatively based on the repu- used. Conservatively, the annualized consultants derive these rates by appears to be a reasonable range tation of sub-standard septic tanks cost to manage and dispose of calculating the actual cost that of treatment savings that can be used in an onsite septic system. retained solids is approximately a treatment plant incurs for the anticipated due to the reduced BOD When planning and selecting collec- US$24 per household, per year. treatment of BOD and TSS. and TSS generated by an effluent tion and treatment options, the val- The 2010 Water Environment The 2010 National Association sewer collection system. ue of the interceptor tank should be Research Foundation (WERF) of Clean Water Agencies (NACWA) Additional possible capital fully understood and measured as performance and cost fact sheets for survey of more than 100 munici- savings attributable to lower- part of the decision-making matrix decentralized systems show that the palities found that the average strength wastewater – as well as of the entire system – not only the operation and maintenance (O&M) BOD and TSS surcharges were savings related to the associated collection portion. costs for effluent sewer and gravity US$0.3112/lb and $0.2588/lb, debt service – can also be realized sewer wastewater collection systems respectively. Review of various rate in investments such as headworks, Author’s Note are essentially equal. This cost resolutions across the US supports clarifiers, aeration basins, solids Mike Saunders is the market comparison includes the cost of these averages. Based on these costs handling infrastructure, and segment leader for engineered managing solids from the inter- – and factoring in the reduction in blowers. Surcharges for BOD and systems at Orenco Systems, Inc., ceptor tanks of effluent sewer wastewater strength – the upper TSS do not include these costs. based in Punta Gorda, Florida, systems. This comparable cost was bounds of potential savings through While this study doesn’t develop United States. Saunders has a confirmed in a long-term cost study use of an effluent sewer collection definitive savings associated with bachelor’s degree in civil engineering performed at the City of Lacey, system would be more than $132 reduced wastewater strength, it and has worked in municipal Washington, US. Accordingly, this per year per residential customer. does provide a simple method to and collection study assumes that the collection The derivation of surcharges for highlight the potential value that for more than 25 years. system costs are equal between BOD and TSS includes both fixed an interceptor tank generates in gravity and effluent sewer, and costs, such as labor, and variable a complete wastewater collection focuses solely on potential savings costs, such as electricity and and treatment process. This generated in the treatment process chemicals. Generally, operational value is often not quantified in a References Day, R. L., Ph.D., Zhu, Y., Ph.D, Pierce, S., & that can be attributed to a reduction savings attributable to BOD and comparative analysis of wastewater Franklin, A. (2008, September). An Examination in wastewater strength. TSS are specific to variable costs. collection systems; however, it of Failing Private Septic Systems in Pennsylvania [Scholarly project]. In PennState Extension. Treatment process savings due BOD has a major impact on costs should be. The value of the tank Retrieved from https://extension.psu.edu/ to lower-strength wastewater from associated with relative to offsetting cost savings in an-examination-of-failing-private-septic- systems-in-pennsylvania. an effluent sewer include reduced processes, aeration equipment, the collection system and treatment Other references used in the development of energy demand, reduced chemical clarifiers, and, to a lesser extent, process is highly variable – but this article are available from the author.

World Water March / April 2019