Recovering a Failed Sand Filter Wastewater Treatment System

Recovering a Failed Sand Filter Wastewater Treatment System By Michael C. Brandt, PE Advanced Designer of Subsurface Sewage Treatment Systems Project Manager MFRA, Inc. All subsurface treatment systems are to be designed based on the standards outlined in Minnesota Chapter 7080, 7081, 7082 and 7083. When designing, constructing and operating these types of systems, many different standards and procedures need to be utilized to maintain successful operation of the treatment system. These systems can appear to function the same way as conventional treatment systems, but they are affected by unique factors that cause them to function differently. As professional designers and operators, we need to understand these unique characteristics in order to have successful wastewater treatment systems with increased life-cycle benefits. This article will take you through one situation of failure when subsurface treatment components are merged with surface water treatment components as well as the differences that allow each component to properly function. One of the key issues to understand while designing a centralized collection system is how the selected treatment device functions. If you are using technologies used under the septic system code, the designer and operator must understand the unique characteristics the treatment system requires as far as TSS, BOD and hydraulic loading. Several of these treatment units are derived from conventional wastewater treatment technologies however they require different operating procedures and pre-treatment procedures. Sand filters are one such device that can be used in conventional wastewater treatment systems, typically, after primary and secondary treatment systems to polish the wastewater prior to discharge. Sand filters are also used in septic-type treatment systems after the septic tank as they provide a media for organisms to grow on. It is critically important to understand the specific keys to designing a proper filter for the proper use. Sand filters are very sensitive to the hydraulic and organic loading rates of the effluent being supplied to them and how they are used in the treatment process. Recognizing the Problem During the recent residential boom, housing developments started to develop their own small on-site systems to handle the developments wastewater needs. Typically,

developments that have a centralized wastewater treatment system use some form of subsurface treatment, yet there are some that have a surface water discharge permit. Depending on the size of the wastewater treatment facility, these systems must be permitted by the local governmental authority or by the MPCA if the average daily flow exceeds 10,000 gallons per day. In 2009, MFRA was contacted by the operator of a 78 lot residential development located in the Township of Big Lake, Minnesota to look into the issue of a failing sand filter, built and designed by another company. This development was platted under the Sherburne County Zoning s Cluster Development ordinance of 2002, which requires parcels of less than 2.5 acres to be connected to a community wastewater treatment system. The system was designed as a 6,500 square foot recirculating sand filter, with a primary clarifier, 19,500 gallon recirculating tank, a final clarifier with Alum addition and UV disinfection. This treatment facility was designed to handle an average daily flow of 19,500 gallons per day of wastewater, with an average BOD concentration of 200 mg/l and a TSS concentration of 250 mg/l. MFRA met with the operator and representatives from the Homeowner s Association to discuss the issues with the wastewater treatment system. During this meeting the operator and Homeowner s Association discussed the problems of water ponding in the sand filter. Immediate recommendation of possible solutions to the issues as well as potential costs and provision of a monthly cost budget for the operations and maintenance of the wastewater treatment system was needed. Water ponding at the top of the filter

Based on the site visit in June and the meeting with the operator, it was concluded that the sand filter was ponding water at the surface and was not allowing water to flow through the filter for proper treatment. The operator had indicated that they had been flushing the distribution and collection lines as well as the flows back into the recirculation tank, with the water seeming to increase each time they flushed the system. During this flushing process, some black matter from the filter had been observed to be entering the dosing and collection lines. The black matter was an indication of the filter forming a dense layer of organisms somewhere in the filter media and causing the media to not allow water to filter through the system. In researching records at the Minnesota Pollution Control Agency (MPCA), it was recognized that the filter was not operating properly as problems started to appear as early as April 2007, with elevated levels of Fecal Coliform, BOD and TSS in effluent. Sand filters can become plugged for the several reasons including, fine particles in the sand which do not allow water to pass through the media, filter fabric is installed between the different materials (which plugs with the finer material) and blocked or broken drain lines in the filter and forming bio-mat, each which subsequently overload the filter. Understanding the Situation To fully understand the issue of the ponding water within the sand filter, MFRA had to first look at the current flows, strength of the flows and the design elements used to design the filter. This treatment facility was designed in 2001 and permitted by the MPCA in 2002. Upon review of the MPCA permit file, the wastewater treatment system used the following design information for the basis of the system: Number of Homes to be served by the system: 78 Average Daily flow: 19,500 gallons per day (250 gallons/day/home) CBOD: 250 mg/l TSS: 250 mg/l Sand Filter size: 6,500 sqft Recirculation tank size: 19,500 gallons Collection system of 2.5 and 3.0 inch diameter low pressure forcemain At the time, there were 59 homes on the system, with the following chart indicating the influent flows and strength of flows from September of 2007. Date Flow (gal/day) CBOD (mg/ L) TSS (mg/l) Phos. (mg/l) Sep-07 8030 951 161 9.0 Oct-07 8694 291 76 8.2 Nov-07 8457 220 145 6.2

Dec-07 7671 362 249 11.7 Jan-08 7923 319 188 8.9 Feb-08 7838 229 82 8.2 Mar-08 11152 280 99 9.0 Apr-08 8077 630 1162 8.7 May-08 7571 363 386 8.5 Jun-08 7750 422 828 8.2 Jul-08 7871 368 156 6.6 Aug-08 7848 563 856 44.6 Sep-08 8473 473 277 9.8 Oct-08 8219 307 181 9.9 Nov-08 8843 372 170 8.0 Dec-08 9271 265 107 8.8 Jan-09 8939 449 673 7.3 Feb-09 9068 216 146 7.4 Mar-09 9368 433 204 7.0 Apr-09 9049 631 487 19.3 May-09 9091 347 118 8.8 Jun-09 9023 510 354 7.6 AVG 8556 409 323 11 The average daily flow from the 22 months of data was 8,556 gallons per day (156 gallons per day, per home) and the average influent CBOD was 409 mg/l. The average daily flow was 43.8 % of the design flow with 74.4 % of the potential homes on the system. Flows ranged from a low of 7,571 gallons per day to a high of 11,152 gallons per day, in May and March of 2008 respectfully. We can see that since November of 2008 the influent flows seemed to level off and maintain consistent flow around the 9,000 gallon per day range. Flows seemed to be fairly typical of developments like this, with the spike in March of 2008 possibly due to a leaky toilet or new home coming on line. For a wastewater system like this, operators will want to see flows at 70 % of the design flow for the sand filter to be operating properly. With 58 homes on the system and an average flow of 156 gpd/home, we documented the influent flows at 62.4% of the design flow given the design flow per home was assumed to be 250 gpd/home. Based on the given information we can see a low value for CBOD to be 216 mg/l to a high of 951 mg/l and the average value for CBOD to be 409 mg/l. These values are higher than expected for domestic wastewater levels but can be attributed to the use of grinder pumps for the collection system and long runs of forcemain with few homes on the system. Grinder pumps are used to grind up the large particles in the wastewater to allow flow into smaller diameter pipes. This grinding action leads to an increase in BOD and attributes to longer settling times for the particulate matter to settle out of the waste stream. The forcemain issue was due to a lack of 100% occupancy in the development.

Because the collection lines were sized to handle a full development, there was not enough flow to allow the system to flush out as designed. This lack of flow can lead to areas within the collection system storing wastewater which allows it to turn septic. Elevated BOD levels can also be due to the increased use of household cleaners, garbage disposals, concentrated time periods of elevated water usage or other owner specific uses. The wastewater treatment system the development uses is a hybrid system, using a septictype treatment system with a surface discharge-type wastewater plant. The surface discharge-type of wastewater treatment facility allows the designer to utilize smaller design flows to size the separate treatment components of the system. Based on the flow usage data compared to the design data, the use of the lower flows had not adversely affected the operation of the treatment system. Since the design of this wastewater treatment system was completed, there have been several changes in the septic system rules in the State of Minnesota. Previously, all septic systems were to be designed under Minnesota Rule Chapter 7080. Over the past year, these rules have been amended to deal with larger septic systems and now include Minnesota Rules Chapter 7080, 7081, 7082 and 7083. In researching the older chapter 7080 and compared to the new 7081, 7082 and 7083 rules, in regards to sand filters, there have been several amendments to the recommendation of media particle size, hydraulic loading rate reduction and checks on the organic loading rate across the filter. The media (sand) has since been modified to include more detailed particle sizes than the previous 7080, while the hydraulic loading rate has decreased from 0.25 gpd/sqft to 0.2 gpd/sqft. Additionally, the organic loading rate continues to restrict the loading rate on a recirculating sand filter and continues to be less than 0.005 lbs of BOD/ sqft. In checking the design based on hydraulic and assumed organic loading rates, the filter had been sized properly. Specific challenges in designing a septic-type system treatment component are in the design of the septic tank or primary settling tank prior to the sand filter. Sand filters are susceptible to clogging in the event of large particles or large organism growths occurring in the filter media. Over the past four to eight years, there have been numerous research studies on the effects on grinder pumps on the ability of particles to settle. This research shows there is a greater need to increase the capacity of the septic tanks, allowing time for the solids to settle out of the wastes stream. These studies have since resulted in significant changes to the approach and design of septic tanks with additional modifications in the rules. They are as follows: Septic/Settling Tank Sizing: October 1999 Chapter 7080.0600 subpart 4 C 2b. Septic tanks: For design flows greater than 1500 gpd, the septic tank capacity shall be 1,125 gallons plus 75 percent of the design flow.

Primary Clarifier Size should be 1,125 + (19,500)*.75= 15,750 gallons. February 2008 Chapter 7081.024 Subpart 2 A (2) For collection systems with grinder pumps the septic tank capacity shall be the average daily design flow * 4.0. Primary Clarifier Size should be 19,500 gpd * 4.0= 78,000 gallons Distribution of Effluent: A properly operating system should distribute wastewater evenly across the surface of the sand filter, in order to have even organism growth in the filter. Upon review of the plans and specifications, it was discovered that the distribution system for each zone needed another lateral with more orifices and that the proper residual head of 5 feet was not part of the TDH head calculation of the pump. Under-Drain System: The re-circulating sand filter had eight zones with an electronic control valve on each zone. Under Chapter 7080 the system should have a drain pipe for each zone, so the effluent will properly drain through the media. The system only had two drain lines for the entire filter, which possibly contributed to the blockage of the media. Having only two drains in the filter caused the effluent to have longer detention time in the filter and also contributed to allowing solids to settle in the filter media. Control System: Re-circulating sand filters should be timed to allow for dosing as well as each section resting between cycles to avoid hydraulic overloading a zone in the filter. In looking at the system, the control panel did not have a timer and the system was dosing based on a float in the recirculation tank. The dosing of the filter was not properly allowing each zone to rest and was letting the water directly flow through the media. Because the zone was not allowed to rest, it became hydraulically overloaded and caused a lack oxygen movement in the filter. The lack of oxygen caused a different type of organism growth which caused the blockage of the media. Operational Issues: Previously, the alum was introduced prior to the final clarifier to settle out the phosphorous prior to discharging the effluent to the surface water. Additionally, some of the operation notes indicated there was a potential the alum could have been added to the primary clarifier prior early on in the start up of the treatment system. This procedure may have introduced Alum directly into the sand filter and could have caused the media to block over time. In observing the operations of the system, the sludge from the final clarifier was transferred to sludge storage tank. This may have been due to the fact that the operator would decant some of the water from the sludge storage tank into the recirculation tank to save on the cost of sludge disposal. It is possible that this practice

could have also introduced trace amount of alum into the system as well and led to the premature failure of the system. Getting Back on Track Once MFRA completed its analysis of the wastewater treatment facility, a report was developed and presented to the homeowners association. The report outlined two alternative approaches to correcting the issues with the sand filter. The first solution suggested that the sand filter media be replaced and more settling tanks to be added upstream of the wastewater treatment plant in order to reduce the solid and BOD loadings to the filter. The second option suggested that settling tanks be added along with another aerobic treatment device to reduce the TSS and BOD along with the replacement of the filter media. Other options for replacing the sand filter media were briefly looked into but were discounted based on the lack of information on successfully recovering the filter media. These alternate methods (for recovering media filters), involved adding air, chemicals or organisms to the filter media for a period of time. Using this method, the filter must be out of service during treatment time and this was not an option for this development. Since there was not a by-pass point for the system and with the system possible taken offline, the development would have been discharging raw wastewater to surface water. Effectually, taking the treatment system off line for an extended period of time was not an option. After the presentation of the report findings, possible solutions and a plan was needed to determine how to proceed with the project. The homeowners association voted to proceed with developing plans and specifications for the first option and worked with the local governments and MPCA to complete the repairs to the wastewater treatment facility. MFRA met with the MPCA, county and township representatives to raise awareness of the situation while developing designs to repair the system. The main concern from the local government standpoint was the complete failure of the system and the repercussion for the residents and several meetings were held at township, county and state agencies to prevent this project moving forward. MFRA prepared the plans and specifications for the system repairs. These repairs included the following improvements to the wastewater treatment system; Addition of five (5) 18,000 gallon septic tanks upstream of the recirculation tank; Replacement of the

recirculation pumps; replacement of the distribution piping and addition of one more dosing lateral to each of the eight zone; Re-programming the controls and addition of a timer to correctly dose the filter and control the valves to only dose one zone at a time. MFRA also collected alternate bids for replacing only part of the sand filter or the entire sand filter and determine other solutions if in the event of the entire filter needing to be reconstructed as well as installing a new collection drain system in the filter. Septic Tank Installation Permits were needed to alter the wastewater treatment system and MFRA worked very closely with the MPCA staff as each party understood the urgency of the situation. Through these efforts, a permit modification was completed in approximately four weeks along with the approval for the plans and specification. The entire cost of the project was estimated at $300,000. Several bids were received and the project cost came in approximately $75,000 under the estimate. Even with project costs under the estimated budget, funding was still a major issue for this project. MFRA worked with the homeowner s association to obtain possible grants and additional funding for the project. For this project, grants and loan options were not available for the for the homeowner s association. Fortunately, the local township agreed to fund the project and assess each of the properties. Securing the funding was a major challenge to this project, but through the entire process a collaborative effort by the township, county and state governments managed to get this project into the construction phase. During the reconstruction process, other issues with the sand filter became more apparent. When the media removal process began, the contractor tried to drain the water out of the sand filter by exposing the under-drain system in a couple of areas in the filter. While excavating the drains, the water repeatedly accumulated in the trench until the top of the drain tile line was exposed. Overnight these areas dried out, but when the excavation began two to three feet from the previous excavation, water still remained in the filter media. This indicated that the lateral movement of the water in the drainage media and sand media was moving very slow or not at all. When the media was dry enough to examine, two very distinct bio-mats were discovered. One bio-mat level

extended three to four inches below the tile and above the distribution piping and the second bio-mat was at the drainage media and sand media interface line. However, the sand media between these two bio-mats looked very clean. The finding of the two biomats confirmed our initial analysis of the filter being organically overloaded and also indicated the potential of alum being allowed to enter the filter media as well. Bio-Mat and blockage, as seen in; Left: Hole near tank and; Right: near water ponding MFRA prepared a new operations and maintenance manual for the project. The manual outlined how the system should function along with key areas to be monitored. The manual is a work-in-progress and should be updated and components are changed out in the system. MFRA was also on site to provide training for the operator on the system, in order to make sure everyone understood the intent of the treatment system and how the components should function. We also discussed where to introduce the alum to the system and that decanted water from the sludge tank should only go to the final clarifier and not any where else in the system. Work began on the project on November 9th, 2009 and was substantially complete by November 25th, 2009. Several other items came to light during the construction of the project as were not originally foreseen. A few examples of this were that the water table was actually higher than anticipated, electronic valves were not functioning properly and the splitting device needed modifications. The final completion of the project came in under the estimated budget and was completed in an extremely timely matter. The system officially started operations the first week of December and met permit requirements for discharge during the first month.

Wastewater Distribution Piping The Final Product The wastewater treatment business is evolving to included smaller package plants as well as larger on-site treatment systems. The key to making these systems successful is in the thorough understanding of each treatment component used in the system; not only from a design aspect but also from the operations side. Because these smaller systems do not offer as much flexibility to the operator as a large scale facility, small changes to the flow or strength of the wastewater can have a huge impact on the treatment device. The designer needs to apply the appropriate codes through out the design of the system and understand how the codes affect the operation of the treatment device. The operator must be trained on how to effectively run the system and provide the correct maintenance measures to keep the system running. If one or a combination of these details is not fully addressed in the design or operation, the system can have a premature failure. There have been some question as to the validity and life span of some of these smaller treatment systems but if these systems are given the attention they need for the proper design and operation they can be a successful treatment option for the smaller residential development. About the Author Mr. Michael Brandt, PE is a professional civil engineer at MFRA, Inc. Michael has a Bachelors of Civil Engineering degree and is a graduate of North Dakota State University. With 12 years of experience in civil engineering and wastewater experience, Michael is a MPCA certified Individual Sewage Treatment Systems Professional. His experience includes operation of municipal water and wastewater treatment facilities and designing large onsite treatment systems for private residential and commercial projects. Michael works in both the public and private sectors and has completed projects for industrial and wastewater clients as well as residential and commercial communities. Some of his experience includes working with the City of Roseau and working on developments such as the Ridges of Rice Lake and the Shores of Eagle Lake. Michael currently is an MPCA Certified Advanced Designer II SSTS and a LEED Accredited professional. Selected by the MPCA, he has served on the MPCA Need to Know Committee and has recently spoken at the National Onsite Wastewater Recycling Association in Baltimore, Maryland where he presented a paper on Decentralized

Wastewater System[s:] From Concept, Through Design, Construction and Operation and Maintenance.