Roediger Questions and Answers
The following is a transcript of some of the questions and answers that we asked Dr. Roediger:
[Chris Eaton] What material are your egg digestors composed of?
[Dr. Markus Roediger] They are made of pre-stressed concrete or steel (we
do not construct the vessels). Below the minimum sludge level is no coating needed, a
poxitar coating is applied in concrete or steel digesters above the minimum sludge level.
[Chris Eaton] What is the ratio of sludge input
to methane output?
[Dr. Markus Roediger] In a good two-stage digestion system, you
may assume that almost 90% of the degradable organic (volatile) solids are degraded. You
may assume that 60 to 65% of these organic solids are degradable (in sewage sludge). You
may assume for the US that 75% of the raw sludge solids are volatile. This leads to a
degradation of over 50% of the volatile solids fed, and of approx. 40% of the total solids
fed. [Dr. Markus Roediger] You may further assume that approx. 0.9 m3
of digester gas is generated per kg of volatile solids degraded (or 0.45 per volatile
solids fed). Methane concentration is typicalle 2/3 of the digester gas, the remainder is
mainly CO2).
[Chris Eaton] First, is there anything glaringly wrong with the
basic design?
[Dr. Markus Roediger] In your anoxic tank you generate a mixture of
CO2 and N2.
[Chris Eaton] Next, is the organization of the building correct,
or should the aeration tank be placed above the final clarifiers? Originally the aeration
tank was placed on the bottom due to it's large size, but we are now reconsidering this as
it would involve added pumping costs. Also, what saftey issues need to be addressed
concerning the hadling of methane?
[Dr. Markus Roediger] You should put the anaerobic digester(s) on
top bacause of safety reasons. Pumping sludge up is a minor energy consumption. Sludge
flow is less than 1% of wastewater flow. Should the anerobic tank and the anerobic
digester be placed on the same level, or would it not be worth the additional pumping
costs?
[Dr. Markus Roediger] You have a very sophisticated system with anaerobic, anoxic and oxic tanks.
[Chris Eaton] Which tank is the "biological (secondary)
treatment"?
[Dr. Markus Roediger] In your case secondary treatment includes
your anaerobic, anoxic and aerated reactors including your clarifiers. You call
these units together an activated sludge treatment system.
[Chris Eaton] What are primary clarifiers?
[Dr. Markus Roediger] Primary settling tanks for removal of solids
forming the primary sludge (mainly consisting of fecal matter)
[Chris Eaton] Methane collection: What types
of pipes do you recommended (even sizes/diameters)? Is there a pressure needed for the
system to operate?
[Dr. Markus Roediger] The pressure is kept slightly above
atmospheric pressure. You should also consider a gas holding tank. Gas pipes should be
stainless steel pipes to prevent corrosion. Size would be 8".
[Chris Eaton] For a 20 million gallons per day plant, can you
approximate the final sludge output?
[Dr. Markus Roediger] Approx. 10 metric tons of solids per
day. You certainly want to dewater this sludge e.g. by means of belt filter presses. You
may achieve a sludge cake having approx. 20% of solids, giving a daily sludge cake
production of 50 metric tons per day.
[Chris Eaton] Can you give us an estimate on how much would it
cost for any of the following:
[Dr. Markus Roediger] Please be aware of the fact that the
following figures are very rough guesstimates, they also include technical equipment:
5,500 gallon anerobic tank [Dr. Markus Roediger] $ 40,000
250,000 gallon anoxic tank [Dr. Markus Roediger] $ 400,000
900,000 gallon aeration tank [Dr. Markus Roediger] $ 8,000,000
500,000 gallon final clarifier [Dr. Markus Roediger] $ 1,100,000
4000 m3 egg digester [Dr. Markus Roediger] $ 4,000,000
39,000 UV contact tank [Dr. Markus Roediger] ???
This is all reguardless of the cost of putting it in a vertical structure, just the cost
for the components.
[Chris Eaton] We need more clarification on the primary
clarifier.
[Dr. Markus Roediger] It is a settling tank to remove the (primary)
sludge that is in the incoming wastewater. The primary sludge from the primary clarifier
plus the secondary sludge (produced biomass) from your biological treatment stage (your
activated sludge system) is called the raw sludge. Raw sludge is fed into the anaerobic
digesters for sludge stabilization (anaerobic biological degration of organic matter). The
sludge from the digester is called digested sludge (or biosolids).
[Chris Eaton] How much would such a primary clarifier cost?
[Dr. Markus Roediger] A 700m3 primary clarifier would
cost (construction + equipment) approx. $600,000
[Chris Eaton] How does a primary clarifier differ from a final
clarifier?
[Dr. Markus Roediger] It is very similar. Final clarifiers are
deeper.
[Chris Eaton] Does the primary clarifier eliminat the need for a
grit removal system?
[Dr. Markus Roediger] No, because you do not want to have the
grit in your sludge (this would cause wear of all the equipment, and grit would build up
in the digesters) .
[Chris Eaton] If phosphorous is removed in the
aeration tank, is nitrogen removed in the anoxic tank? What is the difference
between the anerobic and the anoxic tanks?
[Dr. Markus Roediger] That is a bit complicated.
Biologicalphosphorus removal is done by increased incorporation of phosphorus by the
activated sludge bacteria. They do so if the are alternatively subject to anaerobic and
oxic conditions. The phosphorus is removed with the biomass removed (the so-called waste
activated or surplus sludge). Nitrogen removal is different. The ammonium nitrogen (NH4+)
present in the wastewater is oxidized by special bacteria (the so-called) nitrificants to
nitrate (NO3-) in the oxic (aerated) tank. Nitrate rich water is
recirculated into the anoxic tank (not aerated), where other bacteria use the nitrate as
oxygen source. The nitrate is there reduced to elementary nitrogen (N2) that is
released into the air. An anaerobic tank contains neither free oxygen, nor nitrate, an
anoxic tank contains no free oxygen, but nitrate.
[Chris Eaton] How big would a dewatering facility be that would
be capable of handling 1 MGD dry time flow? and how much would such a unit cost?
[Dr. Markus Roediger] I assume that you have anaerobic sludge
digestion. A 1 MGD plant generates approx. 750 kg dry solids per day. If you want to
dewater 5 days per week, and 8 hours per day, you naeed a dewatering capacity for 130
kg/h. You can assume for digested sludge a capacity of a belt filter press of 500 kg per m
belt width and hour. A single belt filter press with 1 m effective belt width would be
plenty. Assume $200,000 for equipment and installation, and $ 100,000 for building.
[Chris Eaton] Would the primary clarifier be the same size as the
final clarifier?
[Dr. Markus Roediger] No, primary clarifiers are much smaller. Approx. 1
h detention time, for final clarifiers you may assume 4 h.
[Chris Eaton] Is the sludge from both units used in the egg
digestors?
[Dr. Markus Roediger] yes
From: "Christian Eaton" <thekaseman@hotmail.com> Save Address Block
Sender
To: mxr@roediger.com Save Address
CC: colwell@mindspring.com, rcoslow@hotmail.com Save Address
Subject: RE: Research Project at Mainland High School - Daytona Beach, FL
Date: Wed, 23 Feb 2000 06:26:03 PST
Dear Dr. Roediger,
Thank you once again for your speedy reply. You can not understand how much you are
helping us. But now we have more questions.
_____________________________________________________________________
#1: your following comments need a little bit of clarification. Which tank is the
"biological (secondary) treatment"? What do you mean by "100% of the
flow"? What are primary clarifiers? We have grit removal system with a screen and a
centrifuge, is that what a primary clarifier is, or something else?
[Dr. Markus Roediger] You forgot the primary clarifiers. Where you have anaerobic sludge
digestion, you should have primary clarifiers. They reduce the load into your biological
(secondary) treatment stage and increase gas production.
You have a very sophisticated system with anaerobic, anoxic and oxic tanks. However,
please consider that you have return flows (sludge flom clarifiers to anaerobic tank
[approx. 100% of the flow], mixed liquor from aeration basin to anoxic basin (to return
nitrate for denitrification [can be much more than 100% of the flow]).
I would suggest the following set-up. Have the activated sludge system
including the clarifiers) on a single level. If you expand, you build
another parallel system on top. You would need to lift only a portion of the flow.
_____________________________________________________________________
2: Could you please help us with approximate tank sizes (in gallons if possible) and
number of tanks required for a 20 MGD plant? We are having trouble getting the correct
RATIOS of sizes for a specific daily flow rate.
primary clarifiers
anerobic tank
anoxic tank
aeration tank
final clarifiers
digester
gravel pit
UV/Cl2 purification tank
Also, can these RATIOS be scaled to whatever size we need? or is there an optimized size?
_____________________________________________________________________
#3 Methane collection:
What types of pipes do you recommended (even sizes/diameters)? Is there a pressure needed
for the system to operate?
_____________________________________________________________________
#4 sludge produced
For the 20 MGD configuration in #2, can you approximate the final sludge output?
_____________________________________________________________________
#5 safety
If we wanted to put both the a anerobic tank and the sludge digesters on the same level,
would the bottom level be sufficient? or would the top level be worth the added pumping
cost for safety concerns?
_____________________________________________________________________
Thank you again for all of the help you have given us. We have a meeting with the whole
group today from 3pm till 6pm eastern standard time. If you could respond by then it would
be greatly appriciated. If you have AOL Instant Messanger you could contact us durring
that time under the screen name "clhcolwell".
Thank you once again,
Chris.