[Digestion] Environmentalists Anti-AD Issues

[Jim McNelly]

On Tue, Jun 28, 2011 at 2:45 AM, Randy Mott <randymott at ceeres.eu> wrote:

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> Mr. McNelly,  ****
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> We are doing centralized co-digestion facilities in Poland with Danish
> know-how. The wet AD process and local use of 5% Total Solid digestate works
> very well in Denmark and meets all of the market requirements we see in
> Poland. The EU imposes a requirement that farmers have storage for manure,
> so the biogas storage works well for them. We do provide storage and assist
> in application, but this is still a small cost compared to processing
> digestate. Presumably you are getting money for the dry compost/digestate
> material, otherwise it is hard to see how the business model can work. *Until
> there is a corresponding income for the digestate, we don’t see how
> processing it in any fashion can work*. We are evaluating it at one
> location in Poland, but frankly the rest of the project will have to be very
> profitable to cover the cost on the back-end. With 11 Euro cent per kWhr, I
> doubt that it is feasible. What is your cost per ton to process and your
> revenue per processed ton?****
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> When phosphorous prices go up and the value of digestate goes up, the
> market may change. We operate in an agricultural market where organic waste
> is regularly used by farmers, sometimes whether it is legal or not. Thus,
> selling a waste-derived fertilizer is a steep, uphill battle.
>
> ****
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> Randy Mott****
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> CEERES****
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> Warsaw****
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> ****
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Randy,

When I first worked with dairy manure biogas digesters in Colorado in the
1970s, it became clear to me that pumpability, mixing to recirculate scum
and settled solids, and a second stage digestion worked well in respect to
optimizing gas yields. I have also familiarized myself with anaerobic
digestion of biosolids at municipal treatment facilities during years of
operating a land application and composting business. I like wet systems,
particularly simple ones, for manure digestion. Where I begin having
problems is using wet AD for wastewater treatment solids and industrial,
commercial and institutional food scraps.

A rule of thumb is that half of the capital cost of a wastewater treatment
facility is to clean the water and half the cost is to stabilize the sludge.
 If a community is challenged by land application of Class B, PSRP
biosolids, and they would be better served by a Class A PFRP operation, then
I question the wisdom of building a dual treatment system of anaerobic
digestion, then thickening followed by composting.  The new AD system being
built at the Washington DC Blue Plains facility reportedly will get only
enough biogas to pay the interest, hardly touching the capital and operating
cost.  Their objective is to reduce dry solids by up to 35% to lower land
application cost and ensure PSRP, not to generate revenue from biogas.

One facility we built at a chicken slaughterhouse takes 1.5 dry tons a day
of raw, primary solids, mostly blood and fat, from a dissolved air flotation
(DAF) system centrifuged to 15% solids. The wet tons equate to 7.5 tons per
day. To this, the client adds about 10  tons of wood chips for porosity and
moisture conditioning. The poultry operation had quotes from several
engineering firms to build an AD digester for around $2 million. The
composting system, including mixer, loaders, truck and screener came to
around $800,000.  Close to a third the capital cost of the AD system. Farm
scale biogas systems are not so "over engineered" and are much lower in
capital cost, but for municipal wastewater treatment solids or large scale
food scrap digestion, the public typically ends up paying exorbitant
engineering fees, often 40% or more of the total cost. We, of course, sell a
"pre-engineered" system which cuts out the consulting engineers, which
typically puts them into direct competition.

At the slaughterhouse, the resultant 12 tons per day at 40% moisture compost
at 2% N wholesales for around $100 per ton. After AD, the poultry operation
would still have a liquid sludge problem that would require wintertime
storage and a dedicated, non-farmed land application site during the summer.
 Land application costs are estimated at $.05 per gallon for 10,000 GPD, or
an operating cost of $500 per day as compared to income of $1,200 per day.
 We also need to factor in the requirement of 1kg of polymer at $8 per kg
per dry ton to flocculate to 15% solids. Rates of 3 kg of polymer per dry
ton for thickening digested solids are not uncommon, three times as much as
thickening raw, undigested solids.  This is one argument for dry AD over wet
AD.  As I follow the food scrap to liquid AD systems operations, I am
hearing reports of problems with plastics, coconut shells, avocado pits,
cutlery, glass etc. clogging pumps and becoming a land application nuisance.
Inconsistent feedstocks have been a problem, particularly the acidity of
tomatoes.  Put a slug of something odd in a well running AD digester and the
bugs can go into a panic. One farm, I am told, turned away a truckload of
freezer burn chickens, still in plastic wrappings, stinking with maggots.
Yuck!

I have seen similar numbers regarding capital cost reduction percentages of
in-vessel composting vs AD at larger wastewater treatment facilities. But if
a community does not have problems with land application of Class B and they
have already put digesters in place, or the private option to create value
added compost is not readily available, then the cost benefit analysis could
be significantly different. Only when a community is forced to compost to
meet Class A do they consider thickening and composting digested solids, or
if we can reach them, before building an AD system. Most windrow compost at
half of one percent nitrogen sells for around $20 per ton. It requires a
fully aerated compost at 2% N with high maturity ratings to command such a
premium.  Nurseries and greenhouses tend to avoid windrow compost as it is
often anaerobic, full of weeds, inconsistent and comparatively low in
nitrogen. This is where quality control and quality assurance with higher
net N becomes critical.

Another key consideration is that most windrows are actually uncontrolled
anaerobic digesters, using a brief burst of aerobic activity after turning
to provide an optimum temperature environment for anaerobes. It is common
knowledge that a pile uses up its oxygen within thirty minutes after
turning.  Without forced aeration coupled with high rates of air for heat
removal, non aerated composting has a difficult time managing raw primary
solids or energetic feedstocks such as food scraps.  Processing digested
solids that have 30% or more volatile solids already removed is quite
different than solids that still have volatiles in the 90% range.

The capital cost of our containerized aerobic composting system ranges from
$30,000 per ton per day (TPD) down to $15,000, depending on the size of the
operation. The operating costs range around $6 per initial throughput ton
using $1.00 per liter in fuel and $.08 per kilowatt and $17 per hour for
laborers plus a 25% benefits package. The labor is around one operator per
20 TPD for systems under 100 TPD and 1 laborer per 50 TPD at 300 TPD.  Our
main operating cost advantage is that containers in the 22 and 44 ton size
capacity are moved and dumped using container straddlers at 100 HP vs
front-end loader dependent systems that require 250 HP to move material in 4
ton bucket loads, scoop by scoop.

It makes little sense to make relatively high nitrogen aerobic compost and
then discount it into the $20 per ton soil amendment commodity marketplace.
The key is to sell it to nurseries and greenhouses where it competes with
peat moss and bark products, products that often cost up to $200 per ton.
 We are working on a 4% nitrogen biofertilizer product that requires a
fourth as much in the way of bulking materials as aerated compost, and this
product should be able to sell in the turfgrass market, again around $200
per ton. Interestingly, the 4% high N stabilized product requires a dry
anaerobic digestion first stage of volatilization.  We have never seen
farmers willing to pay a premium for 2% N compost, well made or not. Since
the aerobic conditioning eliminates phytotoxicity, the compost is better
marketed for container planting at around 10-15% of the total mix where
significant increased yields are readily apparent. Turfgrass fertilizer
markets are much larger here in North America, which is why we are moving
toward the even higher 4%N product. They do not want dried poultry manure at
6% N if it smells like chicken shit when it gets wet.


-- 
Jim McNelly

Renewable Carbon Management, LLC
44 28th Ave N Suite J
Saint Cloud MN 56303
320-253-5076
www.composter.com


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