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What Can We Do With All This
Food Waste ? C. Merritt, C&S Engineers
Here’s the Deal
How big is this problem- Food is one of the big 4 (water, energy and climate change)
Sources of food wastes
What are the technologies used on food wastes
Follow the economics of a 100ton/day plant through each option
Its all about energy conservation and finding a way to convert the waste to a useful
product.
Sit back and enjoy
How big is the problem?
Types of food wastes
Supply chain waste
Table scraps
Food processors
Waste Food Chemistry
Fruits and Vegetables
Mostly water
Dissolved sugars/ starches in the liquid part
Solids have cellulose, hemicellulose, starches, protein and ash – not a lot of fats
Processing wastes
Diverse but generated in mass quantities by type-just walk through a supermarket
Higher solids content
Can be segregated relatively easy
Table scraps
Diverse and difficult to separate types
Food waste properties
The liquid phase of fruits and vegetables is relatively easy to ferment or
digest
Fruits and vegetables solids do not ferment nor digest easily
Processed food waste high in solids does not ferment nor digest easily, but can
make good candidates for animal feed or solid fuel
Table scrap food wastes are difficult to process into animal feed or solid fuel
Table scraps do make good digester feedstock and good compost feed when
mixed with yard waste.
We tend to lump all three types together and seek to find one technology to
handle the three types .….hard to do
The Hierarchy of Solutions
The fact we have strict food safety
regs, transport a lot of food and we
value appearance ……will always
have food waste
The technologies available
Landfilling
Composting
Industrial uses
Anaerobic digestion
Fermentation
Hydrothermal processing
Use as animal feed or solid fuel
Landfilling: Terrible option for energy recovery
Option1: 100 tons taken to the landfill
each week
10 dump trucks worth of food waste dumps in a landfill
Tipping fee of $40/ton
Cost of $40 x 100 = $4000
Benefits
Convenient – out of sight out of mind
Consequences
No energy recovery
Leachate to deal with
CO2 and CH4 escape into the atmosphere
What happens in the compost pile.
Needs oxygen (>10%) to keep the process aerobic, otherwise
the anaerobic bugs take over and you get nasty odors …like
Hydrogen Sulfide
The process creates CO2 and a little CH4
Needs 50-60% moisture for optimal conditions
Anywhere from 30-50 % mass reduction
The composting microorganisms produce heat and enzymes that
result in organic matter degradation
Compost feed stock is measured in Carbon/Nitrogen ratio with
30:1 being optimal
C/N ratio goes down with compost time
Three stages of biological activity that
breakdown the material and digest it.
Each stage is governed by the temperature
and the bugs that thrive in those
temperatures.
Option 2: Compost 100 tons food waste
per week 10 dump trucks worth of food waste dumps in a composting site
Tipping fee of $40/ton
Costs are $40 x 100 = $4000
60 tons of compost @ $10/ton value = $600 return
Processing cost $ 5/ton = -$ 500
Net income is then $4,600 – $500 = $4100 /100 tons
Benefits
Cheap fertilizer
Mainly CO2 (vs. CH4) produced
Better than landfilling
Consequences
Need land space and method of turning over compost
Still have not recovered any energy
Anaerobic digestion (industrial process)
Biological breakdown of organics in the absence of oxygen. Happens in a land
fill, in your septic tank and in your gut
Can be duplicated in a controlled environment using tankage
Converts organics to methane and carbon dioxide
The methane can be captured, purified and used as a fuel source
Very good way to convert liquid waste high in organics to energy
Not a new technology, be around for decades
Waste water treatment plants
Large farms- manure pits
Becoming an accepted method for treating food wastes
Anaerobic
Digestion Facts
- Sugars and starches digest
fast
- Fats/ proteins digests a
little slower
- Cellulose and
hemicellulose digest slow
- Digestion rates speed up
with temperature
- Digesters like constant
flow and consistent food
source
Anaerobic digestion viability
Recovered methane is used to fuel CHP units, dryers or boilers
Very good way to reduce COD (chemical Oxygen Demand) in waste water from
industrial plants
Methane is 60-65% of the gas (rest is CO2) and may be contaminated with H2S
Has some limitations
Bugs need 80-120F temps.- challenge in winter months
Pretty tight chemistry for digestion- pH, alkalinity, trace metals etc.
Long residency times (days for complete digestion)
High cost ($600/ton install cost); $20+/ton O&M costs
Need consistent loading for good operation
Digester size must account for residence time…ie 100 tons processing will
require a digester capable of storing 1000 tons for a 10 day retention time
Anaerobic Digestion Mass Balance
Each lb of waste food can create cubic feet of 1.7 cubic feet of CH4 and 1.3 cubic
feet of CO2
This equates to about
(3,400 cubic ft methane / ton food)
Each lb of COD added to a digester will yield 2.4 cubic feet of CH4
Each lb of food digested creates about >0.3lbs lbs of digestate (compost material)
Option 3: Digest 100 tons of food waste/week
10 dump trucks worth of food waste dumps in a AD processing site
Tipping fee of $40/ton
Costs $40 x 100 = $4000
The cost to digest 100 tons @ $20/ton is $2000
From 100 tons of digested food we get 340,000 cubic feet of CH4
The amount of heat from CH4 is 3400 therms for a value @ .50/ therm = $1700 or
The amount of electricity generated form CH4 is 25,000 KWH. At $.08/ KWH = $ 2000
The amount of compost is 30 tons with a value of $10/ton = $300
The net value then is 4000 + 2000 + 300 – 2000 = $ 4300/100 tons waste food
Fermentation
Fermentation is the process of using yeasts or bacteria to convert
sugars to alcohol
Methanol, ethanol or butanol
We know it best for making beer or spirits, but is used to make fuel
grade ethanol.
We can only ferment to about 12% ethanol due to its toxicity towards
the yeasts/ bacteria.
Takes a lot of energy to distill and dry it to 99+% useful fuel.
Any waste high in carbohydrates can be fermented fairly easily.
Currently, corn is the major commodity used to make ethanol fuel,
but we could use waste fruits and vegetables as well.
Typical ethanol production plant
Fermentation Mass and Energy Balances
One (1) lb of sugar will produce about ½ lb of ethanol and ½ lb of CO2
We also know the density of ethanol to be .7893 gm/cc
Therefore we can calculate the volume of ethanol produced to be 183cc or .183 liters/ lb sugar
Converting .183 liters to gallons = .047 gal ethanol/ lb of sugar
We also know for ethanol plants that it takes about 30K btu/gal to distill and dry ethanol from beer
We know that fruits and vegetables contain 8-20% sugars
We also know that the solid mass left over can be used as animal feed or solid fuel
Option 4; Ferment 100 tons food waste each week
10 dump trucks delivers food waste to processing plant
Tipping fee of $40/ton
Costs $40 x 100 = $4000
The cost to ferment and purify one gallon of ethanol is about $0.65/gal
100 tons of food waste at 12% sugar content will yield 24,000 lbs of fermentable sugars
Each lb of sugar will produce 0.183 liters or 0.047 gallons of ethanol
Therefore the 100 tons waste will produce 1128 gallons of ethanol at a cost of $733.
Fuel grade ethanol value vary with gas prices, however at $1.5/gallon this plant could
produce $1,692 worth of ethanol
You would also get 5 tons of animal feed at a value of $150/ton = $750
Net value is $4000 + $750 +$1692 – $733 = $5,709/100 tons
Fermentation Limitation
Problem is an ethanol plant costs about $200MM
to build and there is not a good stencil for a
small ethanol plant
You would need to located near an ethanol
plant
Economics do not favor small fuel grade
distilleries
Waste food is not recognized by DOE/ DOT as a
possible transportation fuel source….no
credits. Ethanol plant are reluctant to
acceptable ethanol from food waste
Option 5: Hydrothermal processing
Treatment of wastes using moderate heat (750F ) and high pressure
(3000psig).
Essentially mimicking what take place in the earth underground over millions
of years in just a few minutes.
Uses steam and a pressure vessel to convert wastes into a oily substance
that can be further refined into a fuel source.
Hydrothermal Process
Could this new technology be viable? Likely not right away.
3000psig vessel in any size > 1000gallons would be very expensive to
fabricate.
May not lend itself well to continuous flow.
If it could work, it would be a very good technology
It is to new to even evaluate on a production scale
Solid Fuel or Animal Feed Production
Animal feed
Solids from fermentation process
Dry waste fruits and vegetables
Lots of nutrient in waste food
Craft brewing spent grains, apple pomace, etc
Solid fuel
Solids from fermentation or anaerobic digestion
Dry waste fruits and vegetables
Some processing wastes
Can be mixed with sawdust or other solid fuels
Either way you would likely have to dry to <10% moisture and pelletize (densify)
Option 6: Dry 100 tons food waste/week for animal feed
10 dump trucks delivers food waste to processing plant
Tipping fee of $40/ton
Costs $40 x 100 = $4000
200,000 lbs at 10% solids = 20,000 lbs of animal feed
Dry animal feed is worth up to $200/ ton
Total value is 10 tons x $200/ton = $2000
But we have to dry it to make is storable
So we need to evaporate the other 178,000 lbs of water
To evaporate water from 80F to steam we need about 1100 btu/lb
Therefore 178,000lbs x 1100btu/lb = 196,000,000 btu’s and this equates to 1,960 therms of natural gas.
At $.50 therm we need $980 worth of gas, which does not account for the dryer efficiency losses or the capital costs of the equipment
Theoretical revenue is $5120/100 tons of food waste
What about making solid fuel from
waste food?
The value of pelletized solid fuel is less than animal feed ($150 vs. $200/ton)
The economics look like this $ 4000
$1500
$ 980
$ 4620/ 100 tons of food waste
One advantage this option has is that saw dust of other yard waste could be added to increase volume of solid fuel.
What if we could remove some of the water
and then dry the food waste to animal feed
or solid fuel…..
I did some research at home. I took fruits and vegetables and ground then up and pressed the water out.
I separated the liquid fraction and analyzed it for COD to determine its value as a anaerobic digester feed stock to produce methane for energy recovery
Then I took the wet solids and dried them to see if they would have value as a solid fuel (or animal feed)
Liquid and solid fractions
Results
I found we could remove about 65 % of the water from
the fruits and vegetables and the liquid fraction was full
of easily digestible or fermentable organics.
Vegetable liquid had 58,000mg/L COD
Fruit liquid waste had 140,000 mg/L COD
This means drying costs are much lower and the organics
in the liquid phase could be digested and the methane
produced to provide fuel to a dryer.
Could we develop a closed energy cycle
while producing a salable product?
Mass balance of the new strategy
Start with 200,000 lbs wet food waste @10% solids
Remove 65% liquid
130,000lbs liquid or 15,587 gallon liquid with an average of 100,000 mg/l COD or 388
gm/gallon
Left with 70,000lbs of wet cake which contained 60% moisture
The 70,000 lbs was 20,000lbs solids and 50,000lbs of water
To remove enough water to get to 10% moisture content we need to evaporate
48,000 lbs of water
We then need at least 48,000lbs x 1100 btu/lb = 52.8MMbtu or 528 therms of NG
Good news: From the 15,587 gallons of liquid waste we have a total of 15,587 gal x
388 gm/ gal COD / 454gm/ lb = 13,321 lbs COD/ 100tons food waste
We know digesters will produce 2.4ft3 CH4/ lb COD, so we can generate 31,970 ft3
CH4 which equates to 32 MMBTU
Option 7: Economics of making fuel or
feed
10 dump trucks delivers food waste to processing plant
Tipping fee of $40/ton
Costs $40 x 100 = $4000
22,000lbs of animal feed or fuel from 100tons food waste
11 tons at 150-200/ton = $1,650 fuel or $2,200 feed
We would need 53-32 = 21 MM btu or 210 therms to complete drying
210 therms @ $.50/ therm = $105
Cost to digest 130,000 lbs liquid @ $10/ton = $650
Net income would be 4000 + 1650 -105 – 650 = $4895 /100 tons food waste turned into solid fuel
Net income would be 4000 + 2200 -105 – 650 = $5445/100 tons food waste turned into animal feed
Summary of economics per 100 tons
Landfill…………………………………………………………………………..$ 4000
Composting…………………………………………………………………….$ 4100
Anaerobic digestion ………………………………………………………$ 4300
Fermentation ……………………………………………………….………$ 5709
Hydrothermal Processing ……………………………………………-…..TBD
Animal feed, dried…………………………………………………………$ 5120
Solid fuel, dried…………………………………………………………….$ 4610
Solid fuel/ digestion (Fruits and Veg only)..…………………$ 4895
Animal feed/ digestion (Fruits and Veg only)………….….$ 5445
Summary
Billions of pounds of waste food each year
Currently, landfilling is most used method, but the worst option
Composting is better economically and environmentally
Several Industrial processes can be used to process wastes, but most have
high capital costs and not high returns
Waste fruits and vegetables have value as a feed or fuel product.
Questions
What are the three sources of food waste?
Which disposal method produces the most CH4? Composting or Anaerobic
Digestion
What is the products from fermentation?
What are the conditions for hydrothermal processing (Temp. And Pres.)
What is the limiting costs for making animal feed or solid fuel from waste
food
