[USCC] Using Wastewater Digesters for food waste diversion

[A.Niv]

Answer and clarifications concerning  Nelson Widell confusion.

We are talking about processing of food waste.  Food waste is generated in
households, supermarkets and commercial kitchens in hotels, restaurants,
hospitals, elderly homes, cafeterias etc. Generally food waste in the
kitchen comes from two sources.
The first is the food preparation waste which are peelings, lives, cut off
roots, discarded piecesvof meat, egg shells etc. Amount of this waste
depends on cultural and economic aspects. In many commercial kitchens the
produce arrives prewashed, prepealed, preportioned, or in carton boxes like
juices and blended egg mélange. Then the amount of preparation food waste is
minimal like in the case of  say McDonalds.
The second source of food waste is what is discarded after the meal is
served. Once again in every kitchen the amounts will vary a lot. It depends
on the cook ;-), the amount of serving materials, size of portions and
whether the portion size is constant or adjusted like in cafeteria setting.
The fast food outlets are very problematic because of the serving materials
being used are often plastics and paper.
It is generally assumed that every person generates 200-300 g of food waste
daily. In highly industrialized societies this amount is less but the
packaging materials are more.
Hotels generate roughly from 50-200 kg of food waste per day, restaurants
30-60 kg with big restaurants up to 200kg.  In residential setting it is
accepted that average 1,5 kg of food waste is generated daily by a
household. It is clear that the commercial and household quantities
interchange. If people eat more outside they generate less in the
households.
The kitchen waste, if properly segregated at source, contains 85 % water and
15% organic dry matter with C:N ratio of 10-5:1.  The plate waste will have
the ration close to 5:1 and preparation waste 10:1 to 20:1 roughly.  It is
clear that the moisture is to high and the C:N ratio is to low for
conventional direct composting without adding cellulose and lowering the
moisture.
Treating the waste in MWWT plants requires logistics of  civilized
collection (specialized containers, trucks, frequency), through liquidizing
and mixing with water so the MWWT plant can process it. If the food waste is
15% dry matter and MWWT plant can process 1% material at most we must dilute
it 15 times!. Then this "soup" is subject to aeration. Aeration costs the
electricity to operate the compressors or aerators which is the single
biggest cost of operating the MWWT plant. The large pools must be
continuously mixed by underwater propellers in order to prevent
sedimentation and creation of anaerobic conditions.  After different
digestion processes the resulting processed sewer must be flocculated using
some chemistry (polymers and flocculants). After obtaining the flocculants
it must be strained in presses or filters to remove the dry matter from the
water. The resulting sludge is still a very troublesome material that has to
limed or composted.
So why not just to compost the waste directly and save yourself all the WWT
plant costs and huge amounts of precious water?

WWT plants charge roughly 2-4$/ cu.m for industrial sewer  with high load.
In one cubic meter of the sewer water we have say 1% dm.  So the cost of
processing this dry matter is average 3$ per 10 kg or 300$/ton.  Plus the
cost of water that must be used as irrigation water or discharged adding
pollution to the lakes, streams and ground waters. Plus the cost of
logistics of transport and liquefying the waste. It will easily come all
together  to the level of 500$ per ton.

The rational solution is composting - either in large commercial composting
facility or on site at source. The KWM-100 is ONE OF THE SOLUTIONS at source
cutting down the cost to 6cents per kg or 60$ per ton. Central composting
facility will charge some fee and cost of transport of specialized
containers is on top. But it all is definitely much more economic than
"free" WWT plant operation.

Andrew Niv







>
> I was intrigued by the concept of using existing wastewater digesters for
anaerobically digesting food wastes.While there may be a number of equipment
conversion issues, if the tankage is available,it would seem like something
worthwhile pusuing.
>   However, Mr. Andrew Niv's input confused me.He seems to be saying that
using an existing WWTP digester would be uneconomical  vs. purchasing the
Masstech(TM) process. The web site he refers us to indicates that the
Mastech(TM) processor has a capacity of 100kg. per day. So the theoretical
500 TPD of food waste he uses in his example would require 5000 of his
units. It is hard for me to envision how that is more economical than using
pre-existing, paid for anaerobic digesters at a wastewater treatment
facility.I'd like to hear more from Mr. Niv regarding mass balance,moisture
and cost comparisons. Nelson Widell
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