Passive Decentralized Wastewater Treatment Systems Offer High Performance and Low Operation and Maintenance Costs for Large Projects

By Donald Prince

Introduction

Wastewater treatment system designers often face the challenge of meeting stringent permit limits yet providing a system with minimal operations and maintenance requirements. 

Mechanical treatment systems, traditionally specified in these large and commercial applications are highly engineered and very good at what they do. However, many engineering firms and developers are realizing that there are options in the marketplace today that may be well-suited for these high-performance applications and can be less expensive. Passive, sand-lined systems, including large, decentralized combined treatment and dispersal systems, have demonstrated to engineers, treatment plant operators, and developers that a high level of wastewater treatment does not have to be expensive or require a high level of maintenance. 

How Passive Combined Treatment and Dispersal (CTD) Systems Work

The ability of sand-lined treatment and dispersal systems to achieve outstanding wastewater treatment without the need of mechanical operations may not be obvious to the casual observer. A CTD system may appear like a conventional dispersal field, however, it is quite different. The system has primary treatment followed by the CTD, which has proprietary features and specified media. The bulk of the treatment process takes place in a relatively controlled environment and throughout the vast area of the dispersal field. This differs from most mechanical systems where the treatment area is limited to the confines of a tank. 

In passive treatment systems, the effluent dispersal area (EDA) provides much more area overall and subsequently exponentially more surface area for the attachment of bacteria that naturally purify the effluent. Utilizing this expansive EDA for treatment eliminates the need to enhance the treatment process with blowers or aerators and provides stable and robust performance. Third-party testing has demonstrated that the Advanced Enviro-Septic (AES) treatment system meets secondary treatment levels from at project start-up surpassing the NSF/ANSI 40 Certification allowance of not meeting treatment levels from the beginning of the test for up to 21 days. It also eliminates problems with intermittent use since sufficient bacterial development is present at any given time and makes these systems ideal for seasonal facilities. 

Due to the simplified performance of passive technologies, maintenance is similar to that required of a conventional onsite wastewater system: pump the primary tank as required, maintain the vegetation over the system to prevent woody vegetation from becoming established, and walk the site to observe for erosion or ground burrowing rodents. If required, sampling can be accommodated per permit requirements.  

Considering the comparative costs of a passive CTD system, these systems avoid the use of costly mechanical components and ongoing contracts for periodic maintenance. They also combine the processes of treatment and treated effluent dispersal into a single bed or trench footprint. Generally, this enables a size reduction compared to conventional dispersal systems based upon the high treatment level and low risk of organic clogging to the receiving surface beneath the system. 

Designing Passive Wastewater Treatment for New Residential Communities 

Development of residential communities in areas without sewer infrastructure can be complicated in terms of wastewater treatment system cost containment and long-term performance. Direct discharge of treated wastewater is highly regulated, and treatment and monitoring requirements are often difficult and expensive to meet. Typically, the least regulatory-restrictive method of wastewater disposal is subsurface dispersal. Mechanical treatment systems, while certainly capable, can become expensive in up-front costs, ongoing maintenance requirements, and where an additional dispersal field is required for subsurface disposal. Conventional dispersal fields can be simple to operate but may be difficult to locate and blend into the terrain due to the size requirements. In this case, secondary treatment occurs in the native soil, where biomat development can affect long-term performance. Passive CTD systems offer the best attributes of both mechanical and conventional dispersal field systems. They provide better than secondary-treated effluent reliably to the native soil, thereby avoiding biomat development and resulting in consistent dispersal over the life of the system. Reductions in field sizing afforded by these systems that achieve certified treatment levels, enable easier siting and use in difficult terrain such as sloping fields. 

In addition to use for individual wastewater treatment systems, passive treatment systems can be effectively used for a cluster of homes or a community. In this case, there are two basic options to get the wastewater from each individual location to the centralized treatment system. One option is to collect wastewater at each individual location utilizing individual septic tanks. Solids are removed in the tank and the primary treated effluent is then distributed to a central location. The second strategy is to distribute raw wastewater via sanitary sewer lines, cleanouts, and lift stations to larger septic tank(s) for primary treatment and dosing to the dispersal field. This community approach to wastewater treatment and dispersal, as compared to individual septic systems, offers more versatility in site development and allows the wastewater treatment system to be easily managed into the future. 

Application in Action

Innovative Development Where No Infrastructure Exists: The Cottages at River Hill, W. Newbury, MA

This project is an excellent example of where a passive combined treatment and dispersal system enabled a new residential community development where no infrastructure was available. One of the most pressing problems with a lack of centralized wastewater capacity is its effect on development within the community the facility serves.  However, in areas without centralized sewer, market forces create innovative solutions enabling unique development opportunities.  Such is the case in West Newbury, Massachusetts. 

The Cottages at River Hill project was developed as a “pocket community” of 30 single-family homes.  Rather than develop an entire property with sizable parcels and large secluded houses, pocket communities, or neighborhoods concentrate clusters of smaller homes to foster community connections.  This leaves the majority of the property as an undeveloped, shared space with walking trails, community gardens, etc., enhancing the quality of life for the residents and greater community.

The developers of this innovative project chose an innovative combined treatment and dispersal system for onsite wastewater treatment that would meet the needs of this community. The 30 homes in the project, including 84 bedrooms, have a total design flow of 9,240 GPD. Normally, an onsite wastewater system of this size in Massachusetts would require pressure distribution to facilitate dispersal of the effluent to the native soils.  However, the performance track record of the Enviro-Septic Wastewater Treatment system from Presby Environmental, Inc. enabled developers to proceed without pressure distribution

Due to separation to seasonal high-water table requirements for new construction, the system was designed in an elevated mound configuration that included 2 beds for a total of 6,002 feet of Enviro-Septic pipe configured in 66 rows at 91 feet each. A portion of one of the beds (15 rows) slopes at 6 percent to allow the large field to blend in with the terrain and save on the cost of additional fill which would have been required for a level bed.  Because of the secondary treatment performance of the system, the Enviro-Septic system is approved for a 40 percent reduction as compared to conventionally sized systems in Massachusetts. This enabled a much smaller footprint and subsequently much less disturbance to the natural areas of the property.

Today, the Cottages at River Hill are fully occupied. The community management association reaps the value of a low-maintenance wastewater treatment system, and the residents enjoy the benefits of a tight-knit community along with the open spaces and natural vistas this type of development provides.

Passive CTD Systems Also Excel as Repair Options when Traditional Systems Fail

Often, engineers and designers are introduced to passive CTD systems due to their search for a better solution to replace a failing system and soon realize the value and reliability of combining a high level of treatment within the dispersal field.  As mentioned earlier, size reductions for certified treatment and the avoidance of relying on native soils for secondary treatment of the wastewater, make passive CTD systems a great solution for sites which prove challenging for conventional systems or not suited to the maintenance demands of mechanical treatment systems.

Application in Action

Víctor Hernández Elderly Housing Facility in Aguadilla, Puerto Rico Replaces Failed Conventional Wastewater Disposal System with New Combined Treatment and Dispersal System

A failed system in poor soil conditions in Aguadilla, Puerto Rico caused the facility to pump the holding tank weekly. The situation made clear the need for a better solution with technology that offered a higher level of treatment to protect and preserve the permeability of the soil. Limited area available for a new system and ongoing maintenance expense were both challenges that steered project designers toward a passive treatment system requiring low ongoing maintenance and associated cost. The small footprint of the system was compatible with the land available and was economical to install and maintain. Also, key to the system selection was the need for superior treatment of the building’s wastewater due to challenging wastewater constituencies typical of these types of facilities.

Following thorough review of the project, Sani-Plant of Trujillo Alto, PR selected Presby Environmental’s  NSF 40 and BNQ Certified, Advanced Enviro-Septic®(AES) combined treatment and dispersal system. A 6,000 GPD system was designed with an additional septic tank that included a Presby Maze septic insert to increase the retention of the waste to allow greater efficiency in the primary treatment. Because of the AES system’s rigorously tested capability for high level treatment, the system needed a treatment and dispersal footprint of only 5,609 ft2 (39 ½ ft x 142 ft) with an application rate of 1.07 gpd/ft2. Also, the project required only 2,600 feet of AES pipe in a butterfly configuration with eight serial sections. This saved the facility a significant amount of space.  Additionally, by installing a passive CTD system with a proven record for exceeding required treatment levels and a reduced footprint, the Víctor Hernández Building saved significant short and long-term costs.

Other Large-Scale Applications for Passive, Sand-Lined Treatment Systems

The reliable, consistent, high level treatment capability of these passive sand-lined systems can be applied to other applications which may require mechanical processes to augment the performance of these systems.  For example, adding mechanical pretreatment to these technologies creates increased reliability of the redundant treatment processes.  Redundant treatment may also be desired to deal with the challenges of high strength effluent applications, where one treatment system reduces the strength to manageable levels for the subsequent treatment system.

Another option for these passive, sand-lined treatment systems is to collect the treated effluent by placing the treatment field within a liner allowing subsequent, possibly mechanical, processes such as recirculation and/or ultra-violet disinfection.

Application in Action

Innovative Upgrade of Community’s Municipal Wastewater Treatment Plant Incorporates Recirculation for Denitrification at Blodgett Landing, Newbury, NH

Originally designed in 1959 as a seasonal wastewater treatment facility (WWTF) for a lakeside community in Newbury, New Hampshire, the 50,000 gallon per day (GPD) Blodgett Landing WWTF was not equipped to meet the treatment requirements of the 21st century.

As the population grew and residences were converted to year-round use, effluent testing and groundwater monitoring confirmed nitrate concentrations were a growing problem. Following a successful pilot study to explore collecting a portion of the treated effluent to recirculate through the sludge layer of the existing 34,000-gallon Imhoff tank to facilitate denitrification, plans were made to incorporate denitrification into the planned upgrade to the treatment plant.

Project engineers recommended incorporating Enviro-Septic technology from Presby Environmental into the existing sand filter configuration, to enhance aerobic nitrification and allow for increased denitrification in the recirculation process. The passive treatment performance of the Enviro-Septic technology utilizes the naturally occurring bacterial process eliminating the need for added chemicals, biological additives, or electrical energy. Its subsurface configuration and low maintenance requirements are desirable for any community. For Newbury, this innovative approach allowed the rehabilitation and continued use of the existing Imhoff tank and sand filter/RIBs resulting in construction cost savings.

The upgraded Blodgett Landing WWTF has been operating for over eight years with very consistent results. The Enviro-Septic technology allows the Blodgett Landing Facility to significantly reduce certain wastewater contaminants including Fecal Coliform, Total Nitrogen (TN), 5-day Biochemical Oxygen Demand (BOD5), and Total Suspended Solids (TSS).

Conclusion

Septic system designers, engineers, and developers should consider making room in their toolbox for the simple, reliable, problem-solving capability of passive, sand-lined wastewater treatment technologies. These technologies have been successfully applied to wide-ranging wastewater treatment system applications for the past 30 years. Easily scalable for residential, municipal, and industrial-sized systems, these passive CTD systems can be applied to solve wastewater treatment challenges that meet the various stringent regulatory and environmental challenges facing wastewater professionals today.

Donald Prince is Technical Advisor at Presby Environmental, Inc. He holds a Civil Engineering Degree from Vermont Technical College and is a licensed septic system designer and evaluator. He has provided technical support for AES and other Presby Environmental products since 2012. Presby Environmental is an Infiltrator Water Technologies company. Don lives with his family in northern Vermont. He can be contacted at Don.prince@presbyeco.com.