-----Original Message-----
From: Francis Y. Huang <fyhuang@mindspring.com>
To: Steve Hales <shales@prodigy.net>
Date: Thursday, 27 January 2000 18:56 PM
Subject: Re: Internet science and tech fair

Dear Todd,

I am sorry about taking a while for me to answer your questions. All the
questions you asked are very good and appropriate. The following are my
answers and hope they can help you.

1.    Anerobic digestion of sludge occurs in an environment where no or
very litter oxygen presents.  The organic substances in the sludge,
primarily containing organic carbons and nitrogens, will be converted
into methane (CH4) and ammonia (NH3). Some sulfur-contining organics
will be converted into hydrogen sulfide (H2S) - the rotten egg smell.
Typically, in a sludge digestion dome having available oxygen that can
activate the aerobic digestion of organic matter. The aerobic digestion
end products might be water (H2O), carbon dioxide (CO2), nitrate (NO3),
and sulfate (SO4). Therefore, for the purpose of producing methane, a
closed system without oxygen, and,  of course, anerobic bacteria, are
needed.

2.    There are many types of naturally occurring bacteria in the sludge
or biomass. Roughly the bacteria can be categorized into aerobic and
anerobic. As the name implies, aerobic bacteria requires oxygen in the
process of digesting -"chewing" biomass; while anerobic bacteria require
no help of oxygen. Consequently, the final product of aerobic digestion
will contain oxygen and that from anerobic contain no oxygen as
described before. Now, there is no easy way to tell how many of each
type of bacteria in a sludge. However, we can control the environment to
"promote" or "activate" which type of bacteria to do the work for us.
For example, sealing off the supply of oxygen in a sludge digestion dome
will make the anerobic bacteria active and make aerobic bacteria
dormant. For generating methane from sludge or biomass, we will like to
have active anerobic bacteria to work. Although there are commercially
available anerobic bacteria can be purchsed,  it will be more economical
to take the advantage of naturally occurring anerobic bacteria in the
sludge as long as we know how to activate them.

3.    Current technology for methane generation from biomass can be
found in two areas: animal farm waste and municipal landfills. In both
areas, the biomass is relative dry and rich in biodegradable substances.
In animal farm waste application, the biomass is contained in a sealed
container and keep the temperature relatively high to promote the
digestion process. Typically, there is no need of any flow or mixing
control . However, as the anerobic digestion is taking place, the pH of
the biomass has the tendency to drop till a point at which the biomass
becomes too acidic and the anerobic bacteria die and the methane
production process will stop. Therefore, pH monitor of the system is
critical. Scientist will check the pH frequently and adjust it by adding
more fresh biomass and removing the "digested" biomass out of the
system.

In the case of municipal landfill, there is literally no control at all.
The methane gas is constantly pumped out, purified, and compressed for
use. Since the landfill itself is a huge amount of biomass, we could not
remove or add anything to it. Generally, when the biomass is used up,
methane production will stop, and you might see the entire landfill
actually "caved in".

5.    By definition, biomass or sludge, is a mixture of anything with
organic substances as the predominant ingredients; However, there are
still a substantially large amount of inorganic substances that bacteria
can not "digest". As mentioned in (1) above, the carbons in the biomass
can be converted into methane, nitrogen into ammonia, and sulfur into
hydrogen sulfide, and many other organic compounds we might not able to
identify. Polymers, such as plastics, can not be digested easily
either.  For the inorganic substances,  heavy metals or compounds will
stay in the residue. Some of the heavy metals may be very toxic and
shall be disposed of carefully. As you can see, when the anerobic
digestion process is complete, what left in the waste will be ammonia,
hydrogen sulfide, plastic material, many unknown organic substances,
heavy metals, and, of course, bacteria and possibly parasites.  These
substances are toxic to environment and must be treated properly.

6. In a typical operation, the liquid waste from a sludge digestion dome
is disinfected by adding chlorine to it and is discharged to a river or
ocean with proper permits from the regulatory agents.  The liquid waste
can also be deposed of by a process called "land farm". In the process,
the liquid waste is spread on an open area to allow evaporation to take
place. After repeated spreading processes on the open area, the top soil
is then removed and deposed of in a landfill.  This method creates a lot
of nuisance such as bad smell and possibly spread of harmful bacteria.

In some cases, when the solids content of the waste is high, the waste
can be dewatered after adding flocculant to thicken the waste and then
be pressed on a pressing machine. The dry sludge cake is then disposed
of in a landfill or can be used as fertilizer on non-agricultural areas.

Write me back if you need more explanation or have more questions. Good
luck to your project.

Francis Huang.

Steve Hales wrote:

Dr. Huang,      I am not sure whether or not I have told you this
before or not ,but I am a member of a group of students who has been
working on an Internet project concerning biomass to methane
conversions.  As our project has progressed many questions have arisen
that we are unable to answer on our own.  We are primarily interested
in what happens chemically during the anerobic digestion of sludge.
What types of bacteria are typically utilized and what are their
advantages and disadvantages. In addition we would like to know what
mechanism is used to control the bacterial population? (ph, sludge
flow rate, etc.)    During the bacterial digestion of the sludge what
materials do bacteria use for growth and reproduction and what remains
after the process is complete?  Are the products safe to reintroduce
to the environment. If not, what must be done to them so that the
would be safe? How can we prevent sludge being dumped into  landfills
and leaching into the soil and aquifer? What technolog y is available
and in use already?  What technology is being developed?

Sincerely
Todd Hales