The onsite system serving an Iowa subdivision was designed so most wastewater continues to flow downhill, eliminating pumps and mechanicals.
The Iowa project was a new subdivision for year-round homes, and the task was to maximize the number of homes while not impacting a nearby creek and ultimately the major river it flows into. The team from MMS Consultants in Iowa City, Iowa, and Lynch’s Excavating in West Branch, Iowa, installed a cost-saving solution consisting of biofilters combined with standard septic tanks.
“The river didn’t have a direct effect on designing the system, but it was a limiting factor as we figured out how many systems we could put on the site,” says Trevor Dickerson, the septic designer at MMS Consultants. The MacBride Pointe development is located in Solon, Iowa.
A few hundred feet north of the subdivision is Hoosier Creek, a tributary of the Iowa River. Once it is fully built, the subdivision will have the year-round homes of people who can live near the recreational area around Lake Macbride State Park and have a short commute to work in Cedar Rapids or Iowa City.
Pumping as needed
Dickerson used a combination of STEP and STEG installations because of the rolling landscape. All homes have tanks for primary treatment, and wastewater from each of the 24 housing lots is sent to one of eight jointly owned outlots where final treatment takes place.
Each of the 17 single-family homes on the site has a concrete tank from Wilkinson Precast of Riverside, Iowa. The development also has seven duplexes, and each of those has two tanks. The size of the tanks is governed by the size of the home and the need for a pump. Four-bedroom homes have 1,500-gallon tanks. Five-bedroom homes have 1,750-gallon tanks. Capacities are split with two-thirds of the volume assigned to the first compartment and one-third to the second.
A 4-inch pipe carries effluent to each home’s tank. Tanks provide anaerobic primary treatment. The first compartment separates fats, oils and grease, and settles solids, and the second compartment provides initial treatment. On lots where a rise in the ground prevents gravity flow into the collection pipes, technicians set a 1/2 hp pump, usually from Goulds Water Technology, a Xylem brand, that provides enough head pressure so effluent can reach the collection pipe and flow by gravity the rest of the way. In these cases a 250-gallon pump chamber is a specified addition for the home’s tank.
Most of the pipe was 4-inch Schedule 40. In a couple of cases pipes crossed the safety zone around a well, and that pipe is Schedule 80, says Jason Laing, foreman at Lynch’s Excavating. In some cases the crew used 6-inch pipe as a main collector to cross under a street.
Collection pipes from a group of three or four homes carry wastewater to a custom-made 300-gallon concrete distribution box from Swales Precast, of Strawberry Point, Iowa. These boxes were created with either three or four outlets depending on how many biofilters serve each cluster of dwellings. Biofilters are Premier Tech Aqua Ecoflo units using coconut fiber as the filter medium. Water from the biofilters is discharged to the surface. Premier Tech Aqua’s claimed 10 mg/L of both BOD and TSS meet the Iowa threshold of 25 mg/L for surface discharge.
Lynch’s technicians set standard manholes in green areas for access to the collector pipes. Those are 48-inch-diameter made by County Materials in Iowa City.
Technicians did the subdivision job with the company’s Caterpillar 314 excavator, Case TR320 skid-steer, and Case 580 Super N backhoe. When the job was done they brought in a Caterpillar D6K bulldozer to knock down the dirt piles. A Spectra Precision laser level was used to keep everything in alignment.
Check once, measure twice
It took time in the field to work out placement of the units, Laing says. Technicians made careful measurements to make sure the Ecoflo units and distribution boxes were in the right place, to ensure elevations were sufficient to avoid extra grading yet provided enough elevation for proper and equal flow to the biofilters, and to make sure the biofilter lids were at the proper elevation. The first two distribution boxes went in slowly as the crew worked out its method, but the other boxes were set faster, Laing says.
Because MMS created the entire development plan, the treatment outlots could be drawn for maximum utility, Dickerson says.
“They are shaped to allow for replacement systems if they’re ever needed, and to allow for gravity flow into the biofilters. Aside from mechanical simplicity, this system avoided the complications from using a lot of big pumps and shared tanks, which automatically raises issues about tracking usage and allotting payments. We wanted to make it as easy as possible for the homeowners association.”
The streets were a challenge to the project.
“The streets were already in when we arrived on the site. And the water system was already in, so we had other lines to be careful of as we did the work,” Laing says.
Boring took care of the street problem. A total of four bores done by another contractor were needed to run lines beneath a street and to the north side of the property where the biofilters are placed.
Inside the distribution boxes, Laing handled the need for flow adjustment with a simple solution. Each 4-inch pipe flowing out of a box is equipped with a cap that has a 1-inch hole drilled off-center. Adjusting the flow through that pipe is a matter of rotating the cap.
“Although the recommendation is for some kind of mechanical controller, this is much simpler than any sort of valve. There is much less to go wrong,” Laing says.
Designing for the market
A challenge for Dickerson was sizing a system for a subdivision that didn’t exist, and where people can build what they wish. He looked across the street and at area properties for guidance. He found a subdivision designed about a decade ago and with a shared septic system based on three- or four-bedroom homes.
“And that’s just not what people moving into the area now are building. They’re building four-, five- and even six-bedroom homes,” Dickerson says. So he assumed five bedrooms for each single-family home and eight bedrooms (four per side) for each duplex.
The Ecoflo biofilters were not the original choice for the site. Based on the soil analysis, the county originally preferred sand filters. But as quotes came in, a contractor in the bidding calculated biofilters would be significantly less expensive.
“Due to the quantity of units involved, and the simplicity of installing the Ecoflo filters, the developer saved more than $100,000 on this system compared to sand filters,” Dickerson says. After a discussion, the county allowed them.
Each Ecoflo unit holds 1,891 cubic inches of coconut fiber to treat 750 gpd, and the manufacturer recommends replacing that every eight to 10 years, although use will determine the real interval. Replacement cost and annual maintenance is figured into the fees of the homeowners’ association, Dickerson says. Included in the maintenance is annual pumping of seven to eight septic tanks. All tanks will be pumped on a four-year cycle although individual owners may be required to pump tanks that they fill before the scheduled time.
The subdivision isn’t complete, but thanks to careful work future residents will have no worries that their wastewater will damage the environment of eastern Iowa.