Sludge Watch ==> Different sludge treatments - what do they cost?

[maureen.reilly at sympatico.ca]

Pollution Engineering

May 1, 2006


Harnessing the power of biosolids.

Frewerd, Brian


  Municipal wastewater treatment plants (WWTPs) inevitably generate waste 
solids that require further processing. As wastewater effluent discharge 
requirements tighten, more solids will be generated. For centuries, there 
were simplistic ways of disposing of the material referred to as biosolids 
such as burial.

   With advances made in treatment methods, land application of biosolids 
expanded. The EPA spent years developing guidelines pertaining to    the 
safe and beneficial use of biosolids. The result was the 40 CFRPart 503 
regulations for the use or disposal of biosolids. Included in these 
regulations were clear definitions of Class A and Class B biosolids 
treatment methods.

   Over the past few decades, the production of Class B biosolids created 
limited benefits for municipalities around the country. Furthermore, due to 
a number of regional influences--geographical location, residential 
resistance--municipalities found they have less land available for material 
deposition.

   Class A biosolids have enjoyed a greater public acceptance for land    
application as a result of the minimized risks associated with product 
exposure, given the lower pathogen levels required to achieve 
thedesignation. The form in which biosolids are applied also affects 
acceptability. Land application of dewatered biosolids has greater 
acceptance than liquid land application, and dried biosolids has the highest 
degree of public acceptance.

   Unfortunately, producing a Class A final product that is publicly 
acceptable is a result of having to process the biosolids to higher 
treatment levels, leading to a more complex and costly operation.
   While Class A biosolids present a more desirable product, there are    
still areas not willing to accept any form of land application. Several 
regions of the U.S. have attempted to ban land application entirely, and 
Switzerland has banned it effective this year.

   Regardless of the routes taken (liquid or dry), energy requirements    
greatly impact treatment and disposal costs. Fuel costs directly correlate 
with hauling cost, which can be lowered by reducing the amount    of water 
contained in their biosolids.

   Drying is considered energy-intensive and may become more expensive    as 
fuel costs increase. In order to optimize the drying process, biosolids 
energy can be harnessed and cycled back into the system. Energy recovered 
from biosolids can help pay for processing, thereby becoming less of a cost 
and more of a real benefit.

   Energy recovery systems

   There are many variations of dryers. In some thermal processes, the    
dewatered sludge cake--10- to 30-percent dried solids (DS)--is pumped 
through sludge depositors onto a slow moving belt, where it is dried to 
approximately 55-percent DS before transferring to the second belt for final 
drying to 90-percent DS.

   The energy for the drying process is supplied by a biosolids furnace. A 
general process flow diagram for one such system is provided inFigure 1. 
Heat from the biosolids furnace is transferred to the dryer    process via a 
circulation fan and heat exchangers. A condenser is provided to remove 
evaporated water from the drying air.


   Such processes can incorporate a slow-moving, water-cooled grate furnace. 
Air is blown over and through the grate to insure maximum reduction of 
organic material. Off-gas, after being treated with lime orsodium 
bicarbonate and sent to a bag filter, is then released through    a stack.
   The value of biosolids as energy

   The value of efficiently harnessing biosolids energy can be roughly    
calculated by applying various design and cost scenarios, in this case, 
application of such technology on two WWTPs, at 2- and 12-MGD, respectively.

   Table 1 presents four treatment methods that are common in most WWTPs. 
The sludge yields are typical for plants of the two flows provided based on 
an 85-percent sludge yield and 230 mg/1 BOD concentrationof influent to the 
WWTP. The first treatment assumes a typical digestion process is implemented 
and produces liquid biosolids. The electricity cost for an aerobic digestion 
process is not factored into the economics, but would result in an 
additional credit if it were to be replaced by a biosolids energy recovery 
system (BERS).

   Hauling costs are highly sensitive to fuel costs, which would likely 
cause the most cost increase for biosolids application. The distance 
required to find appropriate sites for liquid biosolids will increase in the 
future thereby raising the costs even more. In this scenario, a 2-MGD plant 
could conceivably pay $900,000 per year for final product handling and 
application, while a 12-MGD plant in a similar situation would have to spend 
more than $5 million.

   The second treatment process involves biosolids dewatering. As canbe 
seen, a four-fold decrease in hauling costs is achieved via dewatering. For 
this reason, most municipalities mechanically dewater their    biosolids. It 
is more cost-effective to remove water mechanically than thermally, but 
mechanical methods have their limitations.

   The third treatment process, thermal drying, is typically implemented to 
reach 90-percent DS, which reduces the amount of material further. 
Generating a dry product increases the likelihood of creating a biosolids 
giveaway program (composting), thereby eliminating further costs of land 
application. However, this does not always develop, so hauling costs may be 
incurred to a lower impact given the water removed. Having the dry product 
will potentially lower hauling costs by 400    to 500 percent over a 
dewatered material but has the negative offset    of fuel consumption.

   The fourth treatment process combines energy recovery along with thermal 
drying. By implementing a BERS, the amount of material to be transported out 
of the plant is reduced, resulting in extremely low hauling costs. 
Furthermore, the risk associated with the cost of fuel sources is removed. 
In the scenario presented above, all of the fuel required to dry the 
material is furnished by the BERS. Specifically, a12-MGD plant that 
generates a liquid product and pays $30/ton for land application could lower 
their annual bill from $2.3 million down to    $34,500. This over $2.2 
million in savings makes payback on the capital investment an easy decision 
to make.


Brian Frewerd from L Kruger Inc.,
a division of Veolia Water Solutions & Technologies based out of Paris, can 
be contacted at
(919) 677-8310, by e-mail at
Brian.Frewerd at veoliawater.com

, or visit
www.krugerusa.com

.
Table 1. Potential Cost Impacts for Various Biosolids Treatment Paths

                                                             2-MGD        
12-MGD
Condition & Assumption                               Plant        Plant

Amount of Sludge Produced, # DS/day          3,500        21,000

Potential Costs for Liquid Biosolids
Weight @ 5% DS, #/day                        70,000      420,000
Application Costs ($30/ton), $/yr           $383,000    $2,300,000
Application Costs ($70/ton), $/yr           $893,000    $5,320,000

Potential Costs for Dewatered Biosolids
Weight @ 20% DS, #/day                       17,500      105,000
Application Costs ($30/ton), $/yr           $95,800      $575,000
Application Costs ($70/ton), $/yr           $224,000    $1,341,000

Potential Costs for Dry Biosolids
Weight @ 90% DS, #/day                       3,890        23,300
Application Costs ($0/ton), $/yr               $0           $0
Application Costs ($30/ton), $/yr           $21,300      $127,000
Application Costs ($70/ton), $/yr           $49,700      $297,000
Energy Used
(1300 BTU/lb [H.sub.2]O), MMBTU/yr           6,460        38,760
Energy Cost ($8/MMBTU), $/yr                $51,680      $310,000
Energy Cost ($16/MMBTU), $/yr               $103,400     $620,000

Potential Costs and Credit Using Recovery System
Weight, #/day                                        1,050          6,300
Application Costs ($30/ton), $/yr             $5,750       $34,500
Application Costs ($70/ton), $/yr            $13,400      $80,500
Available Energy
(6,000 BTU/# @ 90%DS), MMBTU/yr              8,520        51,000
Biosolids Energy Value ($8/MMBTU), $/yr    ($68,160)    ($408,000)
Biosolids Energy Value ($16/MMBTU), $/yr   ($127,800)   ($816,000)