|This decentralized treatment plant
has the capacity to serve 1,500 households.
Editor’s Note: Not quite a year ago we ran a report entitled Decentralized Wastewater Systems, and this update begins where the earlier report ended. Instead of a system to service 150 homes, this report describes a system to service 1,500 homes. Here then, the viability of decentralized solutions to wastewater treatment is being proven at a scale an order of magnitude greater than the earlier example. The vast areas between the simple septic tank that serves a single home, and the massive wastewater treatment plant that services an urban area with millions of homes, is being filled in with solutions at any intermediate scale, thanks to innovative entrepreneurs and continuously improving technologies.
And just as in the example of the single home’s septic system, or a small subdivision’s system to handle 150 homes, at the scale of a 1,500 home small town, the treated water percolates back into the aquifers or provides subsurface irrigation, instead of traveling way downstream to a massive treatment plant – leaking raw sewage into the ground through cracks in the big pipes, mile after mile, often only to then disappear after treatment into river runoff or the ocean. Decentralized solutions not only replenish aquifers and replace irrigation water; they avoid the necessity to install miles of sewage pipe at staggering expense, pipe that ultimately begins to leak.
The libertarian potential of decentralized energy and water solutions is only beginning to be understood, much less implemented, but stories like this, where a new community implements a cost-effective, off-grid solution that is arguably environmentally superior to hooking into the grid, provide inspiration. Often technological solutions auger political changes. Will the powerful vested interests that control our municipalities adapt and embrace decentralized solutions? That they will eventually is a given, but how soon will the public sector easily recognize situations where decentralized solutions to energy and water infrastructure provide the superior option?
What is most encouraging is the prospect of seeing decentralized infrastructure proliferate, allowing existing grid to be upgraded to integrate synergistically – so the public utilities buy and sell water, wastewater services, and energy in a free-market driven, interactive relationship with privately held decentralized installations. – Ed “Redwood” Ring
|Reducers being installed for the air
release valves in drip field return lines.
When six land developers wanted to build contiguous subdivisions on the periphery of Cave Springs, Arkansas, they realized one system to handle wastewater for all six subdivisions would be a more cost-effective solution than six individual systems.
Our solution, proposed to the lead developer, Brett Hash (Northwest Services LLC), required two phases: a 92,400 GPD installation, to be followed by a 320,000 GPD installation. The first installation, called “Fairway Valley Phase 1,” went into operation in January 2008 and treats water from 450 homes.
This first 92,400 GPD system was designed by Daniel Lazenby of ESI Engineering, located in Springdale, Arkansas. The major components are:
- 1,250 gallon STEP (septic tank effluent pumping) systems
- 33,000 gallon equalization tank including two 30-GPM pumps and a control panel
- Moving-bed biological reactor
- Submerged fixed-film aeration unit
- 15,000 gallon settling tank
- 15,000 gallon sludge-holding tank
- 25,000 gallon dosing tank including four 55-GPM (2 horsepower) and two 85-GPM (3 horsepower) turbine effluent pumps
- 1/2 inch pressure-compensated driplines on 2 foot centers including valves and headworks
- Four custom-designed control panels
|The completed treatment plant, occupying a surprisingly small
footprint, just awaits some landscaping to completely blend in.
The system has been operating now for nearly one year. Wastewater flows from the homes to the gravity fed STEP systems, where in-turn it is pumped to force mains varying from either 2″, 4″ or 6″ diameters. These force mains run to a lift station which pumps the sewage through a 10″ trunk line to the 10′x53′ equalization tank. Pumps then send timed increments of sewage into the rest of the treatment system. When flows exceed the 92,400 GPD capacity, the phase two system will be activated.
The operation of the treatment system provides a good example of how decentralized sewage treatment plants can deliver solutions not only more cost-effective than individual home or neighborhood systems, but also are cost competitive with larger scale municipal systems. From the equalization tank, the sludge is pumped into the stainless steel reactor chamber, where aerators create turbulence that tumbles the sewage, creating a moving bed. Bacteria grow on 1″ round plastic disks that are free floating and have honeycomb interiors that allow scouring and slouging. This pretreatment can reduce BOD and TSS by 50-60 percent; the moving bed also eliminates dead zones.
From the reactor chamber, effluent flows next into the submerged fixed-film chamber. Microorganisms digest more sludge, and fine air diffusers mix the effluent, allowing the sludge to settle. Sludge that settles onto the bottom of the tank is pumped into a sludge-holding tank, where liquid that may continue to rise from sludge in the sludge holding tank is in-turn pumped back into the equalization chamber.
|Installed supply and
return lines in dripfield.
As effluent is clarified in the fixed-film chamber, it rolls over a weir into a settling tank where BOD and TSS are further reduced to a level of 15 mg/l, which is compliant with the septic code and ready to go to the dosing tank in preparation for pumping to the dripfield. The dosing varies according to each dripfield since they have different loading rates. The largest dripfield has four zones totalling 100,000 feet of tubing, and is directly beneath a golf course driving range. The dripfield that will be used in phase two will have six zones and will lie beneath a fairway.
Our solution allowed the developers to actually build at a higher density than they would have been able to if they had constructed individual septic systems per home – they ended up building on average 3 homes per acre instead of the originally planned 1.5, and the STEP tanks were installed and connected to the phase one treatment plant as the homes were built. Although the microorganisms in the treatment plant take longer to establish themselves since the initial demands on the plant are well below its capacity, as the flow increases the microorganisms will build themselves up until the plant is at full capacity.
The treatment plant was sited across the parking lot from the clubhouse at the local golfcourse, allowing a relatively central location relative to the six subdivisions and, of course, convenient proximity to the open space on the fairways for the drip systems. The drip lines were buried 10 inches deep with an emitter every two feet. The discharge was designed to be level with the roots to irrigate the grass and enhance evapotranspiration or soaking into the ground.
Once the system was installed and operating, the developers handed the system over to the city.
Tom Bartlett is the CEO of Aqua Tech Systems, specializing in the decentralized approach to wastewater systems and management. Serving a wide range of private and public clients, Aquatech utilizes a collaborative approach with equipment companies, land planners, engineers, private consultants, utility providers, lending institutions and contractors to develop tailored solutions for infrastructure design. Founded in 1999, Aqua Tech Systems and its affiliates are professionals dedicated to providing wastewater solutions for the growing needs of today’s communities, providing the necessary resources to allow their clients to make decisions that are economically sound, environmentally responsible and socially equitable. Based in Arkansas and servicing clients all over North America, Aquatech can be reached at 479-527-9880 and Tom Bartlett can be reached directly at 479-530-7922 or emailed at email@example.com