ABSTRACT | The owners of a multi-unit senior care facility in Vermont were faced with replacing an on-site, soil-based wastewater disposal system. This was not the first time this issue was faced; this particular system had failed and been replaced multiple times over several years. Previous replacement designs expanded the leachfield footprint and used chamber-style treatment and disposal. The replacement systems also failed, causing effluent backups and surfacing within just a few years.
A new approach for a replacement system for the latest failure took a more holistic approach. The focus was on determining an understanding of the root cause of the leachfield failures, then designing a treatment and disposal system that targeted reduction of that impact. First, a wastewater characterization study was conducted that showed the effluent was not responding to primary treatment in the manner typically seen with domestic strength wastewater. A chemical use inventory and historical wastewater quality/flow data analysis established why the existing treatment systems had failed in the past. A pilot test of a different pretreatment technology was then performed to ensure effluent quality would be suitable for on-site disposal.
In addition, the facility owners availed themselves of funding from the State Revolving Fund (SRF) program to help finance these critical upgrades. They now have a properly functioning wastewater system that not only addresses their particular waste profile but also addresses the entire facility’s wastewater treatment and disposal needs now and into the future.
A retirement community in Vermont provides a full spectrum of living options to seniors in a campus-like setting, including independent living apartments, assisted living, and full nursing care. The facility’s water and wastewater infrastructure were permitted and built piecemeal over several years as the campus evolved, with five individual on-site, soil-based wastewater disposal systems being built at the rear of the buildings. Total water and wastewater used at the campus amounts to about 20,000 gallons per day (gpd) [75,700 liters per day (L/d)]. These leachfields occupy essentially all the campus area with suitable soils, leaving no space for new disposal fields.
Four of the five leachfields functioned without issues, but one wastewater system in particular proved to be problematic. This system accepted sewage from the assisted living facility which houses the campus’ commercial kitchen that furnishes all the meals served and where wastewater flows amount to 6,000 gpd (22,710 L/d). The original septic system consisted of a number of narrow leaching trenches which failed in the past. This original field was replaced with a new set of leaching trenches located adjacent to the original location, but this new system failed again in a few years.
The third leachfield for the facility was placed in the same footprint as the previous replacement leachfield since no other location was available. This third system used a state-approved pretreatment technology with fabric-wrapped chambers placed in mound sand. This system also failed after only a few years of use. The owner subsequently implemented the recommendations of the system vendor to attempt to rejuvenate the system, but it fouled yet again in 2018. All of these failures were believed to be caused by the leachfield and pretreatment chambers clogging. After this most recent failure, the main field was taken offline, with effluent being collected in temporary holding tanks and pumped out on a weekly basis. Costs for this service were thousands of dollars per month, an untenable situation for a non-profit facility.
Faced with yet another failure and mounting disposal costs, the owners contracted a wastewater engineering firm to evaluate what actions should or could be taken to correct their wastewater issues and prevent them from recurring yet again in the future.
WASTEWATER CHARACTERIZATION STUDY - FINDING THE CULPRIT
A review of the site history and uses provided evidence that the facility’s effluent was not typical domestic strength. With this in mind, the recommended first step was to obtain the data necessary to understand the makeup of the facility’s wastewater.
Results of the sampling results are provided in Table 1.
A set of composite samples was taken throughout the existing wastewater collection system for the assisted living facility discharging to the failed leachfield. Sampling locations included a manhole receiving domestic wastewater, grease tank influent and effluent from the commercial kitchen, and septic tank influent and effluent containing both kitchen and domestic wastewater. Each sample was collected with an autosampler over a 24-hour timeframe and analyzed for biological oxygen demand (BOD), total suspended solids (TSS), nitrate, nitrite, ammonia, total kjeldahl nitrogen (TKN), and oil & grease.
To determine flow and the chemical makeup of the effluent, facility personnel monitored and recorded total water usage, softener backwash flow, and individual kitchen component usage (dish washer, preparation sinks, pot sinks, etc.). The neighboring facility also provided a summary of the types and volumes of chemicals used annually. Active ingredients in the bulk of the chemicals used were surfactants (soaps), caustics, alcohols, glycol, sodium hydroxide, and quaternary ammonia compounds (“Quats”).
Typical domestic strength effluent has a BOD of 250-300 parts per million (ppm) [250-300 milligrams/liter (mg/L)]. Domestic strength effluent was assumed and used as the basis of design for the prior disposal systems. BOD levels were found to be nearly 600 ppm (600 mg/L) and oil & grease levels over 30 ppm (30 mg/L). The collected data also showed that very little reduction in BOD and oil & grease concentrations occurred in the septic tank, which is not typical.
An evaluation of the chemicals used at the facility was performed to determine if one or more could be causing the reduced primary treatment effectiveness. Most of the chemicals used are readily degraded by the bacteria present in wastewater systems. Surfactants can impede TSS partitioning, however, the sampling results indicated that TSS reduction is occurring in the septic tank.
Quats on the other hand are not readily biodegraded. Quats are stable and very effective at disinfection in high concentrations. These products are used to comply with regulatory requirements for disinfection and are a key component of their kitchen cleaning processes. Research into Quats toxicity and function indicated that anerobic degradation of nutrients is inhibited with 5-15 ppm (5-15 mg/L) of Quats present. Significant inhibition of functions such as nitrification can occur with as little as 2-5 ppm (2–5 mg/L) Quat concentration. The reported inhibition of biologic treatment is intensified by shock loads (i.e., occasional high-concentration doses) of Quats.
Review of chemical usage logs indicated that shock loads of Quats could be occurring. These records, combined with the flow logs, provided the information needed to estimate Quat concentrations in the septic tank. Estimated Quat concentrations ranged from 4-18.4 ppm (4-18.4 mg/L), which is above the level at which biologic activity is inhibited. The high end of this range is nearly 10 times the concentration at which biologic activity inhibition had been observed. The presence of Quats appeared to be the cause of the reduced primary treatment efficiency. Quat presence also limited biologic treatment in the pretreatment chambers and soils of the leachfield, likely resulting in clogging and failure of the system.
Removal of Quats from the wastewater stream would result in a significant reduction of effluent strength and increased biologic treatment of the effluent. The owners had researched use of alternative disinfectants, but only chlorine-based products were suitable by regulation. They had previously used chlorine-based products but had abandoned them due to significant impacts on workers and clients due to eye, skin, and nasal irritation. Therefore, no alternate regulatorily acceptable disinfection alternative was available. Quat use had to continue.
Thus, the impact of Quats had to be overcome to reduce effluent strength to levels that would not result in leachfield failure. The engineer recommended a pilot study using actual effluent to prove that potential pre-treatment systems would be effective.
TREATMENT SYSTEM PILOT TESTING
The goal for pre-treatment was to overcome Quats’ biologic activity inhibition by creating highly advantageous conditions for aerobic biologic growth. Aerobic treatment systems are very effective at reducing BOD concentrations of 98% or better. The results of the wastewater evaluation were shared with several vendors. A treatment goal of under 30 ppm (30 mg/L) for BOD and total suspended solids (TSS) was established, which is the tertiary treatment standard of the Vermont Department of Environmental Conservation (VT DEC) wastewater rules. Ideally, BOD and TSS would be reduced to 10 ppm (10 mg/L) each. If the pilot system could bring BOD and TSS levels to 10 ppm (10 mg/L), the replacement disposal field would be essentially dispersing clean water and allow three times the amount of effluent per unit area to be applied to the ground surface. As open space was limited, the goal of achieving tertiary strength wastewater was a key objective for the full-scale system design.
A trickling filter-based technology vendor was selected and asked to perform a pilot test to treat up to 300 gpd (1,135 L/d) of side-streamed effluent. The pilot treatment system was set up in May of 2020 (see Figure 1).
Installation of the pilot treatment system
Two plastic septic tanks were arranged above ground, adjacent to the existing septic tanks. A flexible impeller pump was installed above grade, with a small-diameter PVC pipe and foot valve to keep the system primed. Effluent from the septic tank was drawn by the pump and fed into the two pilot tanks. These pilot system tanks contained plastic media suspended above the effluent. Recirculating pumps sprayed effluent over the plastic media to promote biological growth and treated effluent was then discharged to the frac tanks.
The pilot test ran from May to July and produced favorable results (see Figure 2). Approximately 300 gallons (1,135 liters) of effluent were passed through the treatment system per day. It took approximately 21 days for the biologic growth on the media to become sufficiently robust to result in meeting the 30/30 treatment goal. Adjustment of the flow rate and recirculation volumes were performed to optimize pilot system operation achieving the desired 10/10 for several weeks.
The successful pilot test indicated that treated effluent quality was such that replacement of the soil-based disposal field in the same footprint as previous fields would be possible. A drip disposal field was proposed which would maximize the amount of effluent able to be discharged in the limited footprint. The existing chamber-based disposal system and impacted soils surrounding it would be removed and replaced with mound sand prior to shallow placement of the drip disposal field.
THE SOLUTION
During conversations with the facility owners on the pilot study results and next steps, sewer infrastructure of the entire campus was discussed since all of the wastewater systems are over 20 years old. The owners’ vision for the campus included improvements to increase the standard of care for its residents, so it was a logical next step to consider the other four disposal systems and whether it would be beneficial to build a single system that could serve the entire campus for the next 20 or more years.
Evaluation of alternatives indicated that the proposed pre-treatment and drip disposal system could be scaled up to accommodate all campus wastewater flows. An entirely new wastewater system would not only solve the immediate issues with the facility’s effluent but also ensure that the whole campus would have a well-functioning system into the future. It would also consolidate the permit-required inspection and monitoring requirements. The increase in capital costs appeared reasonable considering the ongoing upkeep and operation of the existing aging systems.
The permitted flows for the entire campus are 20,000 gpd (75,700 L/d). Through the use of the data provided by the pilot study and collaboration with the treatment system vendor, a campus-wide pre-treatment and disposal system was designed. The pre-treatment system uses the existing primary treatment (septic tanks, grease interceptor, pump stations) at each building and discharges to two equalization (EQ) tanks. Two treatment trains of five trickling filter tanks running in parallel provide treatment prior to discharge to a second EQ tank. Treated effluent is pumped from the second EQ tank into either the new drip disposal system designed on the footprint of the former disposal field, or two stone-and-trench leachfields.
The two existing leachfields were selected as they were functioning properly, located adjacent to the drip field, and are oriented in a way that optimizes use of the available space. The discharge pumps for each of the three disposal fields were configured on their own timer-based float assemblies, allowing the operator the flexibility to fine-tune flow rates to each field. The operating goal is to have a uniform effluent flow across the linear footage of the combined disposal systems, which minimizes groundwater mounding beneath the fields.
A new process control building was designed to house the control panels for the treatment system, along with a set of suction lift discharge pumps that circulate effluent to the drip irrigation field. Figure 3 illustrates a process flow diagram of the new system.
PUBLIC FINANCING FOR A NON-PROFIT
The campus now had a solution for the repeated failures of their wastewater system, as well as a comprehensive wastewater treatment system for the campus’ future. However, the necessary capital for the full-campus system was not available, even after the owner conducted an exhaustive search for funding options. One option presented was the Vermont State Revolving Fund (SRF) program, run by the VT DEC Water Investment Division (WID). With tens of millions of dollars in federally assisted aid, it is a key financing program for municipalities to be able to bring water and sewer projects to fruition by providing low-cost loans for engineering, construction, and administrative costs. Typically, the users of the SRF program are cities and towns, but certain non-profit organizations (like the one described here) can be found eligible.
In 2021, the wastewater engineering firm advocated for the owner’s eligibility for the program, noting the health hazards of a non-functioning septic system for senior citizens. After a series of meetings, the VT DEC WID found that the owners were indeed eligible to participate in the SRF program. However, this did not mean that groundbreaking could occur immediately.
In order to use SRF funds, the WID requires an additional level of environmental and technological review. This includes not only an alternatives analysis to ensure the most cost-effective technology is chosen but also a review of environmental land use including wetlands, archaeological research, and prime agricultural soils analysis. Construction documents must be prepared according to the standards of the Engineers Joint Contract Documents Committee (EJCDC), and a specific bidding/procurement process must be followed. This required additional time to address, meaning that final approval from the VT DEC for the project to go out to bid wasn’t received until the summer of 2022, nearly two years after pilot test completion.
Figure 3. The drip irrigation and pre-treatment systems during construction
CONSTRUCTION AND INITIAL RESULTS
Construction of the project (Figure 4) kicked off in the fall of 2022 but was subject to delays due to supply chain issues for electrical components and other building materials. The process control building was built by the end of the year, but construction of the treatment system and drip disposal field had to be postponed until spring 2023. Thanks to a bout of good weather, the contractor was able to push through and complete construction of the system by summer 2023 when the commissioning process began.
COMMISSIONING
The initial step in the commissioning process was establishing the biologic population necessary in the treatment trains to fully treat the effluent prior to discharge into the disposal fields. Considering the history of multiple failed wastewater disposal systems, the engineer and owner were hesitant to discharge partially treated effluent to the new drip irrigation field during startup. Therefore, frac tank storage and offsite disposal was continued for a few more months. Samples were taken regularly from the two EQ tanks upstream and downstream of the trickling filters over the summer to determine when sufficient quality effluent could be sent to the disposal fields. Establishment of biologic treatment proceeded relatively quickly, and results in late August showed that the system achieved the design effluent goal of less than 10 ppm (10 mg/L) for both BOD and TSS. Refer to Figure 5 for a summary of the results.
SUMMARY
By gathering operational, water quality, and chemical use data at the facility, the presence of residual disinfectant in the campus effluent was found to be the cause of multiple wastewater treatment system and leachfield failures. Pilot testing and optimization of proposed pre-treatment methods provided the assurance that the system would meet performance goals of 10/10 BOD/TSS. The pilot study also provided sufficient data to allow for design of pretreatment for the entire campus. High quality effluent is now disposed of via a new drip irrigation field coupled with two existing leachfields to provide a single wastewater collection, treatment, and disposal system.
The pretreatment system is flexible and robust and able to compensate for varying flow rates and strengths of effluent. The control systems in the building allow for remote adjustment of recycle ratios in the trickling filters, balancing of wastewater flow to the drip irrigation field and former leachfields, and data collection for required monthly permit reporting.
Shane Mullen, PE, CPESC, is a Technical Specialist at Weston & Sampson in Waterbury, Vermont. He earned his BS in Chemical Engineering from Clarkson University and has over 20 years of experience in wastewater disposal, potable water supply, stormwater, and site/civil engineering.
Published in NEWEA, Summer 2024.