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fixed film in AD
Posted: Mon Dec 31, 2007 1:20 pm
Hi Bob. Wonderful site you have created. I like the simplicity of your two stage design. Would adding fixed film media in the second stage reduce the HRT, or leave the HRT unchanged and produce more gas? Does it make sense to go to two tanks when the size increases to say around 1000 gallons? And my last question, for now, is can mixing of the second stage be achieved by placing the heating coil in a tube so that it starts a convection current? Thank you in advance for any help you can provide. Angus. PS Happy new year.
Posted: Sat Jan 05, 2008 7:08 pm
Adding fixed film media wouldn't actually add hydraulic retention time, but it should achieve a similar effect by giving the microorganisms something to hang on to. That said, though, I've found fixed film media to add cost and complexity. But of course there are all kinds of media, some more expensive than others. One thing to keep in mind is, how will you deal with it if/when it comes time to empty the digester. It is one thing to pump out sludge and liquid, but something else again to remove, clean, and/or dispose of the media.
Re number of tanks... In general, a single tank will be most cost effective, (having less surface area to insulate and lose heat through, and requiring less floor area and connecting piping and fittings). However, there may be valid reasons to go with two or more smaller tanks, rather than one large one, including redundancy (if one tank fails, the other(s) can continue), and better use of available space.
Yes, I have seen designs that achieve mixing by convection loops. But I don't know how well it performs -- or compares with other methods.
Posted: Tue Jan 08, 2008 3:44 pm
I'm curious as to what volume the AD calculator uses for horse manure?
My friend has a horse farm with 20 horses. Her horses produce 54 cu ft of manure and bedding per day. If the bedding is 50% and there are 20 horses then the average daily output is 1.35 cu ft.
If I use your AD calculator, the recommended tank sizes come nowhere close to 12 days HRT (3 days HRT acid phase + 9 days methane phase).
I don't know if I'm missing something. One suggested reason for the difference might be averages that exclude pasture manure. Her horses are kept mainly in stalls. Her horses average around 1000 lbs and she says that there is little difference between summer and winter.
I thought that if I knew what volume of manure the AD calculator uses for horses then I can apply a correction factor. This way I can get more accurate estimates of gas etc.
Posted: Tue Jan 08, 2008 4:52 pm
The online calculator
waste composition data is taken from Other Homes and Garbage
by Leckie, Masters & Young. A screenshot of the lookup table is posted in this topic
. (Scroll down to to near the bottom of the thread.) It shows 30 lb/day for horse manure. At an estimated 60-65 lb/cu ft, (depending on moisture content) this would work out to about 1/2 cu ft/day average, or about 37% of your measured volume which includes bedding.
Another caveat occurs to me. If the waste material includes un-shredded straw, it is not readily pumpable, which suggests even more, a hybrid batch/continuous system.
Posted: Tue Jan 08, 2008 6:07 pm
I like the hybird systems that I have seen on the web. I think there is potential to operate them as businesses. Accepting waste from the community and turning it into products and energy for consumption. Wow.
But for now I want to build an AD digester for my friend that will help her in her business. Her bedding material is wood shavings so it should help increase the C/N ratio. In fact it may be too much for the 30 to 1 recommended.
The wood shavings adsorb the urine from the horses. How does urine affect the operation of the digester?
I look forward to the warmer weather when I can get started on a design for her operation. I am spending this time gathering as much info as possible, but I know that there is nothing like doing it to really answer all the questions.
Posted: Tue Jan 08, 2008 6:19 pm
Are you familiar with the USEPA's Agstar program? They have some design software and a comprehensive handbook free for downloading, that may fit your application better than my smaller scale stuff.