Presented by Mike Stephens, Business Manager
Stephens Consulting Services, P.C., Haslett, MI 48840
INTRODUCTION
Current sand filter technology was introduced in Michigan in 1994 and quickly gained popularity as a pretreatment method when site conditions were difficult. Sand filters have effectively been used where native soil conditions would not have provided adequate treatment; or more commonly, where native soils did not have adequate natural permeability for the use of a traditional soil absorption system. Typically, engineers and other consultants have been called in to design onsite systems using alternative technology --- sand filters being one of the choices.
After preparing custom designs for 100 or more sand filters over the past 6 years, and watching those systems being installed, our firm has found that the process could be improved through the development of a modular system. Over two years ago our firm designed and developed the Modular Sand Filter.
BASIC TREATMENT PROCESS
Traditional sand filter designs and custom design sand filters use the same treatment processes the majority of the time. Each sand filter is preceded by a septic tank providing primary treatment of the wastewater. A second tank used as the dosing tank frequently follows this, or sometimes the second compartment of the septic tank is equipped for dosing the sand filter. Most systems are equipped with an effluent filter and/or a screened pump vault. A timer to provide micro dosing to the filter, where the wastewater moves down through 24" of sand, controls this pump. The final treatment stage occurs in the soil absorption field. The Modular Sand Filter (MSF) system utilizes all of these same treatment processes.
CONSTRUCTION
Traditional sand filter designs come in various sizes, but most are approximately 360 sq.ft. in surface area . They are generally assembled into a single unit, but can be split into separate cells if space is limited. The sand filters utilize plywood walls, a pvc liner, and rubber boots that are assembled to form the filter frame. Typical media quantities that are placed in the sand filter are 28 c.y. of sand and 16 c.y. of stone. The filter contains a 4" underdrain. If required, a final disposal pump vault is installed inside the sand filter to utilize pressure distribution. A network of lateral piping is laid out on the stone surface of the sand filter using small diameter orifices and orifice shields for distribution of the effluent. The filter is final graded with well-drained soil and seeded, or can be left with a stone cover for enhanced air infiltration.
In comparison, the MSF is 22 sq.ft. per module with 4-5 modules for homes with 3 to 5 bedrooms (88-110 sq.ft. in total). The modules can be placed in a central location or divided into individual locations if space is limited. Because the total surface area is reduced in the MSF, the quantity of media is substantially less. The MSF uses 8-10 c.y. of sand and 5-6 c.y. of pea stone for the total system. The MSF polyethylene tanks with rubber grommets provide for both the structure and water tightness required, however, the MSF collects the effluent with a 2" underdrain (see diagram) and transports it to a pump tank outside the filters or directly to the final dispersal area. The MSF uses a nozzle and chamber distribution assembly instead of a lateral and orifice assembly. The 1" distribution assembly mounts on top of one chamber 75"L x 34"W x 12"H, with two 120¯ full cone spray nozzles per chamber. Much like the lateral lines in the sand filter, the chamber is laid on top of a 2" layer of pea stone above the sand layer. With closed plates on each end, it is then back filled with pea stone only to the top of the louvers on each side, with the chamber having void air space inside. Each spray nozzle (8-10 total) will apply effluent to a surface area of 7.9 sq.ft. when mounted at 11" above the surface of the pea stone. These modular tanks, distribution assemblies and chambers are pre-plumbed in the manufacturing process, therefore leaving minimal assembly to be done by the installer in the field.
INFILTRATIVE SURFACE & LOADING RATES
It is recommended that intermittent dosing be utilized for all sand filters. Small volume doses spread throughout a longer time period will create a film flow over the sand particles . This method provides adequate air infiltration and biological production within the sand media.
Typical sand filter designs consist of 72 total orifices being applied to an orifice shield surface area of 0.07 sq.ft. The total infiltrative surface area being utilized then totals 4.8 sq.ft. for a sand filter with a surface area of 360 sq.ft. The MSF using the full cone nozzles applies effluent to 63-79 sq.ft. of a sand filter with a surface area of 88-110 sq.ft. Basing the loading rate on the total surface area of the filters for a 3-bedroom home, the MSF loads the filters at 4.0 gpd/sq.ft. while the traditional sand filter design loads the filter at 1-1.5 gpd/sq.ft. When you calculate the loading on the actual infiltrative surface area being applied effluent, the loading rate for the MSF is 5.6 gpd/sq.ft. while the traditional sand filter design totals 75 gpd/sq.ft. This ratio applies only to the infiltrative surface without any consideration for ponding or lateral surface flow.
MEDIA SPECIFICATION
Our firm has traditionally used the same sand specification for all of our custom design sand filters. The MSF specification was patterned after this same specification, but was revised to reduce the amount of fines allowed in the sand media (Table 1). In creating a standardized design, it was critical to also provide some extent of protection against a marginal media being used in the installation. It was decided that the larger-sized particles in the MSF media specified would reduce the potential for that error.
EFFLUENT QUALITY
Based on limited data, the effluent quality from the MSF is similar to typical sand filter design effluent. Testing is still being done on existing installations as well as new construction. To date, only 7 ammonia results are available, 9 results on BOD, TSS, and fecal coliform, and 11 results on nitrate levels.
DESIGN CHARACTERISTICS
In Michigan, engineers/designers/consultants are called upon to prepare construction plans for most alternative systems, sand filters included. These plans are customized for each site and generally include a site plan showing locations and elevations of all the crucial elements. Our firm found that we generally use the same designs again and again, changing only the site plan and the pump sizing for the system.
Our firm believed that these key elements could be addressed while providing a standardized set of drawings that could be used on most any site. The drawings detail both intermittent and recirculating modular sand filter systems using either a pressure-dosed or gravity drainfield for each. The plans include a table for site elevations and crucial system elevations, a table for drainfield sizing recommendations, a table for health department specifications to be input, and a graph showing acceptable forcemain sizing for different static heads. These plans are used by our staff and by trained installers to submit to the local health department for approval.
As health departments become increasingly familiar with sand filter technology, the decision process becomes routine and focused on the same two issues: final disposal location and pump sizing. When a site has been evaluated as acceptable for a sand filter pretreatment system, we believe that a pre-engineered design allows the regulatory authority to focus on the key elements that vary from site to site. The regulatory agent can dictate the sand filter and drainfield locations, verify the pump meets the design criteria as shown on the plans, and permit the design without significant involvement by the system designer. This can greatly reduce up front design delays and costs to the homeowner.
CONTRACTOR TRAINING
A very important additional component of this process is periodic detailed training of installation installers. Each winter, our firm hosts one or more detailed full-day training sessions for installers and regulatory staff. Covered are the principles of sand filter design, operation and maintenance, as well as the specifics of the Modular Sand Filter.
These trained installers then have the opportunity to install the MSF by entering into an agreement. We provide each installer with signed and sealed engineering plans of the MSF to be submitted to gain a sanitary permit. We ask that each installer agrees to maintain the systems he installs, and we provide technical support for both the installation and maintenance. These are the only installers who can use these plans and install the MSF system. We see this as a significant quality control measure.
The design is easily installed by a knowledgeable installer. The tank and other components, including dosing pumps, tanks and controls, are pre-plumbed and pre-assembled to the greatest extent possible, before delivery to the site, to simplify installation. Being modular, the number of modules can be chosen depending upon the expected flows.
CONCLUSION
As often as designs need to be customized and altered to submit for approval, there is opportunity for error. As permit review authorities receive numerous sand filter designs with differing detail each year, there is opportunity for oversight and error. As installers shop for the specialty components required from various vendors and suppliers, there is opportunity for error.
Pre-engineered designs and pre-assembled systems provide opportunities to reduce oversights and errors. Standardized maintenance programs can be enhanced through standardized designs and systems. Pre-packaged systems have also furthered the opportunity to reduce errors in component purchase errors. Introduction of the Modular Sand Filter has proven to be a tool to standardize sand filter installations, both from a design and a construction standpoint.
| SEIVE SIZE | MODULAR SAND FILTER % PASSING |
CUSTOM DESIGN SAND FILTER % PASSING |
| 3/8" | 100 | 100 |
| No. 4 | 100 | 77/100 |
| No. 8 | 95 | 53/100 |
| No. 16 | 25 | 15/80 |
| No. 30 | 5 | 3/50 |
| No. 50 | 0 | 0/1 |
| No. 100 | 0 | 0/1 |
| No. 200 | 0 | 0/1 |
| D10 | 0.73 mm | 0.4-0.9 mm |
| Cu | 1.9-2.0 | 1.0-4.0 |
2 Small and Decentralized Wastewater Management Systems, Crites & Tchobanoglous, 1998, The McGraw-Hill Companies, Inc., p. 709.