Tuesday, May 4, 2010

A short comparison of advanced wastewater treatment systems

Three main concerns clients express when exploring options for onsite wastewater treatment systems:
  • What is the initial cost?
  • Is the system reliable?
  • What are the O&M Costs?
In reality O&M costs and system reliability are opposite sides of the same coin.   First consider the two main classes of onsite systems: 
  • Fixed film
  • Suspended growth
Fixed film systems include Eliminite, textile filters, peat bogs, sand filters, upholstery foam and others.   Suspended growth systems do not have nearly the wide variety of bright ideas as the fixed film systems and are really just aerated septic tanks.  

If you ask people familiar with the industry you will find that suspended growth onsite systems are notoriously unreliable.  I am not arbitrarily slamming these systems, it is just a fact that when you use an undersized septic tank, dump into it all the liquid, solids and grease and bubble air into the resulting "soup" you are going to have problems.  The only positive aspect of these aerated septic tanks is that they are cheap!   But, once an owner has one of these installed, there is a reasonable probability that the permit limits will never be met.  The unreliability of suspended growth onsite systems has caused many manufacturers to turn towards fixed film systems.

The first Eliminite was installed in early 1994.  At that time most of the advanced onsite systems were suspended growth systems.  There were no textile filters...the main purveyor of these systems was busy selling sand filters.  Foam upholstery cubes were unheard of and most of the peat was still in the bog.  I am being facetious to a degree but you get my point, the industry was dominated by suspended growth systems.  Maybe this is one reason why there was such a huge push in the 1970's and 1980's to sewer everything; it was a well accepted concept that onsite systems simply did not work.

Fixed film systems changed that.  But, what is the most important thing that has to be provided to a fixed film system for it to function?  Well wastewater of course...second most important thing then?....AIR.  If air cannot get to the biofilm, the system is going to have problems, period.  Now think.....Imagine laying on you back and having someone shovel two feet of saturated peat on your face.  Can you breathe?   Probably not, and you have the benefit of a diaphragm muscle expanding your chest cavity to allow air to be forced into your lungs in response to atmospheric pressure.   Microorganisms growing on the surface of the wet peat do not have diaphragm muscles.  How about textiles?  At about 1/2 inch thick these fabric sheets also become saturated with liquid effectively blocking the free diffusion of air to the biofilm.  Eliminite uses our proprietary MetaRocks as the medium to which microorganisms attach.  When dosed with wastewater,  MetaRocks allow the development of a thin liquid film across their surface.  A film sufficiently thin to allow efficient oxygen diffusion into the biofilm.  But that is not all....the size and shape of the MetaRocks allows air to move freely and if we go back to the example of trying to breathe through wet MetaRocks you will see that the large pores allow free air movement.  In fact, even as the biofilm develops on the surface, the pores remain open and available to unhindered passage of air.  This is not the case with the other fixed film media.  Think about it.  Their tiny pore spaces quickly become closed off due to inevitable biofouling.  The only thing to do then is to pull the filter media out and clean it.   MetaRocks do not require cleaning.  So back to the point,O&M.

Do you suppose that maintenance costs will increase if the saturated, dirty media has to be removed and cleaned regularly?  Will the maintenance costs be lower for a system that has already considered the potential for biological fouling?  Why do some systems have such large lids/covers?  These big covers are expensive and a manufacturer would not add them (and their additional cost) unless they were absolutely necessary.  But they are not necessary to get the media IN, they are necessary to get the media OUT for cleaning or replacement.   Eliminite MetaRocks were designed to address and overcome the issue of biological fouling.

Look at the photo of the MetaRocks in the tank (click on it to enlarge the photo) and notice the huge interstitial openings.  These wide open passages allow air to move freely.  They also accommodate a wide range of hydraulic loading meaning the system can handle varying waste strength and flow.  We measured the hydraulic capacity of the media at over 200 gallons per ft^2 per minute.  At that rate flushing (even though I have never seen a situation where it has been necessary) is as easy as opening a valve.

Another really useful feature of MetaRocks is that they are lightweight and free-flowing.  This means that you can build larger systems by using larger tanks.  Think about that for a minute and compare an Eliminite system treating 5000 or 10000 gallons to a peat system or textile filter.   The Eliminite system can be built in one tank, two tanks, you choose.  The peat and textile configurations are only available as a daisy-chain of little toy pods strung together.  Which option sounds easier and less expensive to build and maintain:  a single central treatment plant or a spread out maze of little parts and pieces?

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