Anaerobic Methane Gas: Using Low-Solids System

Generally, sludge is dewatered before going to the anerobic digester process where solids decompose to produce methane. It is this precursor step (dewatering) that achieves a higher solids concentration.

By not dewatering the "sludge" first, you will be need a digester tank volume 10 times as large (0.06/0.006). This will be more costly and very inefficient. No methane can be produced from the water so it is important to reduce the water content unless cost does not matter.

The decomposition will still proceed, but it may be expensive. You will need to heat the digester tank in order for the reaction to proceed. That will be costly.

Thank you eriew for your post, explaining that "water does not produce methane, but the organic matter does". While appreciating the fact that more space and energy would be needed to help methanogenation, the question is:

1. Are there any practical systems now in existence at this low solids levels?

2. How do we estimate the "energy' needed? Or would it be sufficient to have the system kept at a sufficiently "higher" temperature?

This subject has come up as we have a set up in which the solids content (all organic matter, excellent for methanogenation) are less than 1%. It would be extremely difficult to "filter" these solids, although the process would not be impossible. Further, such systems are being studied in tropical (Indian) surroundings, where the summer temperatures are of the order of 32 dg C to 38 dg C, and winter temperatures would be about 9 dg C lowest at night (yet day time temperature would be about 26 deg C or more)

Thank you again

Best wishes

pvhramani

I'll speculate as best I can, the options as I understand the situation.

1-dewater or desludge here's a links to a unit probably oversized, http://www.aaronequipment.com/UsedEquipment/Centrifuges/Decanter/523970.html

2-settling, most things you can separate with power, will separate in time.

3-flocculation probably using calcium and or aluminum compounds.

"Are there any practical systems now in existence at this low solids levels?"

Yes, but the CH₄ output is low relative to the size of the system due to the low organics concentration.

"Or would it be sufficient to have the system kept at a sufficiently "higher" temperature?"

Maintain the temperature between 35^oC and 50^oC. Optimum is about 40^oC. Too low, and the process slows down, and if your reactor utilizes thermophilic bacteria, they may die. Too high, and the bacteria will die. pH level should also be maintained between 6.8 & 7.2, preferably at 7.0. Below 6.5 or above 7.5, the methanogens will die.

"This subject has come up as we have a set up in which the solids content (all organic matter, excellent for methanogenation) are less than 1%."

Such low organics content is usually treated aerobically as it results in a rapid, substantial BOD/COD reduction. Anerobic systems are normally used for high TOC content wastes as it would be very expensive to treat them aerobically. Is there any particular reason why you are treating it anerobically e.g. CH₄ production?

Hello Dvader1000!

Thank you for your nice explanation.

Please see the thread:

http://cr4.globalspec.com/comment/181539/Re-APIX-Pilot-Plant-Design-Project-Recycling-Wastes-Mission-and-Goals

http://cr4.globalspec.com/comment/181606/Re-APIX-Pilot-Plant-Design-Project-Recycling-Wastes-Mission-and-Goals

We have a "problem" posed to us ... to "Do something to convert Wastes". And, our APIX-SEP group Goals/ Objectives are founded on this very concept of creating Sustainable Village Development, using WASTES. The situation here, for example, calls for the conversion of "Waste Water, combined with human Wastes" ... about 57, 000 L of water + 800 Kg of human Toilet wastes. This particular "case" is in an industrial unit employing about 300 people, working three shifts and operating 365 days (effective, 300 days). This could be followed by a system that would combine the Wastes of about 100+ industrial units (all in a cluster), situated within a bout 300+ ac. While the single Industry system may generate electricity to the tune of 40, 000 KWHe, that in the "combined" set up could generate about 1.75 million KWHe. These are based on potential methane gas generation.

Thus, it is imperative that we design/ develop a system that could efficiently convert the "low solids" sludge/ mixture, through anaerobic process into Methane gas. The final "sludge" matter, along with the plentiful water content could be used in agriculture operations as combination bio-fertilizer and irrigation water.

Best regards

pvhramani (hari)

The "Gober Gas Plants" in India has been actively producing methane for cooking as well heating purpose in Indian rural areas.

It consist of a well constructed with adjustable head. The cow dunk and water is added to the well everyday. The methanogenation process takes place producing methane causing the well head to rise, and the methane gas is fed into the boilers to generate heat and electricity as well used as cooking gas.

You may want to check that process.

The possible design of the methane Gas generator system could be Cylindrical, and constructed in Steel ... followed by Epoxy Coating. The 30 day "resident" material systems could be accommodated in a "gas-Holder" type design, as shown below:

The illustration shows a small "dome" shape at the top. The Costs of this system, with the different appurtenances would not be too high, and the space taken up is also not high

Members may please comment on this

Best regards

pvhramani (Hari)

Here's the basic design, referred to elsewhere. I would think that we would be using a series of tanks connected together with 30cm or larger pipes. The incoming tank(s) set up in a standard cascading arangement with the relatively clear graywater being drawn off for other uses and the sludge being transfered to probably 3-5 'methanogenic conversion'chambers of a slightly more specialized design. The tanks would either be buried or enclosed to facilitate the regulation of heat & to fit in with the visual aesthetics of the campus

I should mention,

Can green waste [or other materials ] be gathered and added from nearby rural or residential areas?

Hello Garthh

Yes! These are possibilities that could be considered, depending on how the industrial unit that wants the "disposal" done. We may even consider combining some sawdust ... but are there any experiences/ case histories using sawdust?

Best wishes

pvhramani (hari)

Hi. Sorry for the late reply. It's not advisable to use sawdust as cellulose takes a very long time to break down, so your reactor may be filled up very quickly. How about using the sawdust to grow mushrooms first, then, when it can no longer produce mushrooms economically, throw the residue into the reactor? By then, the cellulose would have been broken down into simpler organics that the reactor can handle.

Have you considered grinding up raw food scraps that cannot be eaten or other organic matter into fine particles, then using them to feed the reactor? By increasing the organics content within, you'll get better biogas output. Just remember to maintain the pH at 7.0.

Provisions for sludge inlets, waste sludge outlets, supernatant outlets, and gas removal will be required. The tank bottom should be conical.

Could the liquid possibly be used to grow algae or "pond scum", to be converted into biodiesel?

Hello

Good thinking! But then, how would we achieve economic, environmental and social benefits, by doing so, in this particular case?

We are trying to take advantage of the WASTES generated by about 100+ Industrial units that create "wealth" to the tune of about US $1 billion (or more), and convert the said wastes to benefit the nearby dilapidated villages, where men and women do not earn even US $500 per annum!

Bio-Diesel "experiment" seems to have once again shown that any development scheme benefits only the minority "richer" sections; the majority "poor" would have to merely be satisfied with philosophies and rhetorics!

Biodiesel production could be a positive thing, it is simply more fuel for production/consumption by whomever you choose to receive it. If the processes are properly controlled (from a philosophical view, not the electromechanical view) then the beneficiaries can be chosen to your standards. I know what you mean about benefitting the wealthy while the poor must be consoled by the rhetoric. One of our philosophers once stated that just because the rich man gets his ice in the summer and the poor man gets his in the winter does not mean that the average is fair for both.

hfello,

i should probably start a new thread, but thought i would ask my question here. i don't know the solid content of my digester. perhaps some of you could hazard a guess. my digester is a plug system, running off of a single flush toilet for a single family home. right now 2 people. toilet flushes 1.6 gallons per flush.

i realise this is very unscientific, but i just don't feel like weighing my solids contents.

i am assuming that they are in the 6% to 15% range. anybody else want to "weigh in on this one"?

It varies between 0 and 100 percent measured to the outlet. Just after pumping out, the solids content is zero. As wastewater is added solids from the raw wastewater will settle in the tank. The liquid will leave the tank and go off to the leaching area. If the tank is never pumped out it will eventually fill up with solids. As more solids are added, they will not be trapped in the "digester" but will carry over into the leaching area and this will eventually fail as these solids will plug up the pore spaces in the field.

You have a strong wastewater entering the tank since you are using a low flush toilet (1.6 gallons). Strong wastewater contains 1200 mg/l of total solids or about 0.12 percent (since 1 liter contains 1,000,000 mg). But this gets diluted by other water such as bath and dish washing. With water conservation, the average per capita consumption of water has declined. Perhaps you are using somewhere between 20 and 50 gallons per capita per day. You indicate a 2 person household. Multiply this by 2 for the average day.

This is the wastewater flow into the "digester" tank. A baffle-wall will allow wastewater to leave the tank over the baffle. However the solids, except those that are dissolved, settle into the "digester" and anerobically decompose. The dissolved solids aerobically decompose in the leaching field.

Grit, inorganic solids that settle cannot decompose in the "digester". For this reason, you may need to pump out the tank eventually depending on the tank volume.

hi,

thanks for the reply. i am confused though. is that 12% or 0.12%? also, i divert all grey water around the digester.

1 percent is 10,000 mg/l so 1,200 mg/l is about 0.12%. One liter has 1,000,000 ppm (part per million) so 1,000,000/1,000,000 = 1.0 or 100% since decimal times 100 gives percent. One ppm is 1 mg/l for WATER. So 1 ml of WATER weighs 1 mg. 1 ppm is equivalent to 1 mg/l for WATER.

So, 1 liter of water weighs 1 kilogram. One gallon of water weighs 8.34 pounds since 3.75 liters equals 1 gallon.

"One gallon of water weighs 8.34 pounds since 3.75 liters equals 1 gallon."

Looks like a rounding error to me - a gallon of water = 4 quarts, a quart = 2 pints, and a pint = 1 pound, so that gallon is really an even 8 lb. But these things will happen when converting from one system to another. Yet another argument in favor of consistency, I guess...

There are 7.485 gallons in 1 cubic foot of water. This volume weights 62.4 lbs. Therefore 1 gallon of water weighs 62.4 / 7.485 = 8.34 pounds. There happen to be 16 ounces in one pint, or 32 in one gallon. But one pint does not weigh 1 pound. These are measures of volume (pint, quart, gallon), not weight. A gallon (US) of water does not weigh 8 pounds.