Hi all,
I previously posted this in brief on a recent thread about biogas in schools, but am happy to elaborate further here!
The case study I would like to submit is RTI International's cocopeat biofilter secondary treatment system at two schools in Muntinlupa City, Metro Manila, Philippines. This technology was developed by RTI through a Phase 1 Gates grant that involved field and lab trials in Southeast Asia. The main pilots were these schools in Muntinlupa, where the local city environmental officer played a key leadership role in championing, designing, and implementing them. I myself did not play any role in their implementation, but was a consultant to RTI following their completion - assisting in their analysis and in considering potential scale up of the idea. All credit goes to my former boss, Mr David Robbins of RTI, and to Mr Jet Pabilonia, the environmental officer of Muntinlupa City. I am simply writing this up to flag your attention to it because I don't think either are active members of this forum! For more information, you can visit the webpage that Dave designed for this project, at:
watsanexp.ning.com/page/cocopeat-biofilter-for (Note that free site membership is required to view the content)
I will focus specifically on the system installed for the Putatan Elementary School in Muntinlupa City, as that was the one I actually visited.
Firstly, what is cocopeat? Cocopeat is the fine powder that remains from dried coconut husks after you remove their fibres (which themselves are useful in other purposes, such as rip rap netting). This is usually done by a 'decorticating machine'. Coconut husks - especially cocopeat - are generally considered a waste product, yet this study found that cocopeat is actually an effective medium for aerobic secondary wastewater treatment, due to its absorptive capacity and the natural variations in grain size that create air pockets in the medium. (See the aforementioned website, and contact Dave, for more details on the various lab studies they carried out to demonstrate BOD/TSS/coliform declines.)
Given the billions of waste coconut husks produced in tropical countries like the Philippines, it is easy to envision how cocopeat use for wastewater treatment could be scaled up (though more difficult in practice due to the transport economics involved). There are a couple local Philippine companies that sell bags of peat already, for various uses, at extremely affordable prices (less than 50 cents per cubic meter) - it is really just the trucking costs that are the problem for larger systems that may be far away from a local cocopeat source!
Turning then to the school, the elementary school contains ~2000 students and 78 teachers (though I don't think that all of these students attend at the same time - Philippine low-income elementary schools often run in shifts), who make use of a number of pour-flush toilets that were donated by USAID after the 2009 typhoon (the area is prone to significant flooding during typhoons). These toilets drain to a 3-chamber septic tank. Greywater from sinks, however, just drains to a nearby canal. The 3rd septic tank chamber then contains a small, 1/4 horsepower flotation pump that generally turns on about 8 times per day for 5 minutes at a time during normal school use (~40 minutes per day @ 125 Watts per hour leads to an electric cost of about US $1.60 per month - very affordable). This pump lifts the septic tank effluent to the above-ground cocopeat biofilter.
This particular biofilter measures ~2.4m x 2.4m x 1.2m and is constructed of fibreglass-coated plywood, which was a cheaper option than using a waterproof plastic liner of the type used in landfills. At the bottom of the box is an effluent pipe that lies in a few inches of coarse gravel, which the cocopeat then sits on top of. The influent pipe enters at the top of the cocopeat layer and discharges the effluent equally across the available surface, thus creating a vertical flow system. A layer of coco fibres / coarse peat is then added over top this influent piping in order to prevent foul odours (this was discovered after feedback from the teachers prior to this top layer being used). No lid is needed unless heavy rainfall is a constant occurrence or there are concerns about children playing in it (here, the unit is simply fenced off).
These dimensions and construction materials are totally flexible though, the only general design rule is that about 1 square meter of cocopeat filter area is needed for each cubic meter per day of effluent flow - more cocopeat than this is less efficient, while less risks overloading the peat such that aerobic treatment ceases to take place due to over-soaking. Given the above dimensions then, this unit is capable of handling about 6 cubic meters per day of flow, which has been sufficient to date.
In another element of sustainability, the effluent pipe from this unit empties to a drip-irrigation system for the school's garden, though could also just empty to a canal.
O&M for this system is minimal. Depending on flows, the cocopeat should last at least a couple years (given the pilot nature of these studies, we still aren't really sure how long it can last, but at least a year or two for sure, quite possibly much longer), whereafter the old peat should be shovelled out (into the garbage or compost) and replaced with fresh peat. It could be adequate to simply change the peat whenever the septic tank is desludged. The pump adds a bit more complexity to the situation if it breaks down, but given its very low usage and simple setup, it will likely outlive the rest of the unit! That said, the school can always contact the Muntinlupa environment office if any support is needed. A pump could also be avoided by excavating the biofilter into the ground, such that the septic tank effluent could just flow into it by gravity.
I personally like this biofilter idea for schools because it can easily handle the variable flow rates that make schools so difficult for other types of systems. Even if the school is not in use for 2 or 3 months in the summer, the cocopeat will not decrease in treatment capacity upon return (unlike, say, a planted gravel filter, whose plants might die if no flow occurred for 2 months). It will simply dry out (or not, if rains are common), then begin to reestablish its aerobic biofilms once flow begins again.
This summary of mine really doesn't do this system justice, so I encourage the authors of this new compilation to also check out the website I listed above and to contact Dave directly for more information - he would be happy to share it. The website also has a great many photos that can be used.
Please just let me or Dave know if you have any further questions!
Cheers,