Why
is the "concentration" of solids important? I was recently
asked this question in an email from an Operator-in-Training on
the east coast. Before we explore the benefits we should probably
define what we will call "concentration" or "thickening"
of a solids stream.
Definition
Generally speaking, "concentration"
or "thickening" refer to:
a) increasing the solids content, and reducing the total volume
of, a wastewater treatment plants solid stream such as primary
sludge, or secondary sludge; and
b) in water treatment, sludge being removed from a sedimentation
process.
Goals
As an operator our process goal is to:
a) remove the solids from a water treatment stream, and concentrate
those solids (and in many cases, find a beneficial use for these
treated solids process stream products) and
b) also reduce the capital and operational costs whenever possible.
Math to Illustrate the Benefits
Some math will quickly illustrate the benefits of solids thickening.
My first example is from a fully nitrifying activated sludge wastewater treatment plant, with an average daily flow rate of 8.2 million gallons per day (MGD). (Okay, I don't have to look further than out the window of my office to find this facility!) Last month, we wasted an average of 0.141 MGD of waste activated sludge to the dissolved air floatation thickener (DAFT) at a concentration of 7,023 mg per liter. The DAFT produced a thickened waste activated sludge of 0.02821 MGD at a concentration of about 3.5 percent solids (a fairly good historical value.) [Our wastewater treatment plant's effluent 30 day average, without tertiary filters, was 3.6 mg/liter suspended solids and 4.0 mg/liter B. O. D., and a Total Coliform medium value of less than 2.2 .]
We will first set up a "pounds formula"
for the average daily pounds sent to the DAFT:
(0.141 MGD)(8.34 lb/gal)(7,023 mg/L) = 8,258.6 lb/day sent to
the DAFT
How many pounds did we capture in the DAFT?
Here, we will use another pounds formula:
(28,210 gal/day)(8.34 lb/gal)(0.035) = 8,234.5 lb/day captured.
By subtracting the two values, we can find
the pounds of solids that went back to the headworks:
(8,258.6 fed) - (8,234.5 lbs captured) = 25.1 lbs returned to
the headworks
What is the percent capture rate? We use
the formula:
IN - OUT x 100 = percent capture (removal)
IN
(8,258.6 lbs/day) - ( 25.1 lbs/day) x 100 = 99.7% captured (removed from the stream)
8,258.6 lbs/day
While this is 99.7 percent capture rate
is within the expected and published a range, it is a 0.3 percent
above are historical average. Our historical sludge volume index
(SVI) values are below 100, which contributes greatly to this
high capture rate. We have therefore taken our 141,000 gallons
per day, 0.7023% solids waste activated sludge, and reduced it
to 28,210 gallons per day, of a 3.5% solids sludge.
What is the percent reduction in volume?
(141,000 gal/day) - (28,210 gal/day) x 100 = 79.9 %
141,000 gal/day
Okay, this is a 80% reduction in volume. Obviously, the benefits are now visible: We do not have to further handle the 112,798.6 gallons/day of water in the solids treatment stream. We now get to send 28,210 gallons/day of 3.5 percent solids to the anaerobic sludge digesters, and 112,798.6 gallons per day of water is sent back to the head works of the treatment plant for treatment. This results in far less cost to construct the digesters (does not have to be much larger for proper treatment), but also lowers the costs to heat the digester, and treat the much larger volume of water in the dewatering process! (We use the methane produced in the anaerobic digesters to heat them.)
MATH EXAMPLE
#2
In reality, the weight of 1 gallon of sludge does vary from the
8.34 pounds per gallon of water, as the concentration of solids
in the gallon increases. (I think this would be a good topic for
a future operator notebook. I will also discuss polymers, lime
addition, etc. that would contribute to this weight per gallon
deviation.)
The benefits of solids thickening are clearly seen in the following example where we will take a sludge solution of 60 gallons containing 1 percent solids (10,000 mg/L). Note below in the math, there are 5 pounds of solids in the 60 gallons. If we thicken the 60 gallons from 1% solids to 5% solids (and also capture of 100 percent of the solids), we have reduced the volume 80 percent, from the original 60 gallons to 12 gallons. This is a reduction of 48 gallons. This is an excellent return on our investment!
By concentrating or thickening these solids further, (and again, to keep it simple for now, by capturing 100 percent of the solids) and thickening from the 5% to 15% solids we remove an additional 8 gallons of water. Our return is not nearly as dramatic as before!
If we place our 15 percent solids cake from our belt press in a thin layer in the hot sun, and the increase from 15% solids to 50% solids, we will remove only 2.88 additional gallons of water. not very much in comparison!
The following is the math I used to generate the above discussion:
Math Work
Setup for 1% solids, and 60 gallons to start off the comparison
with:
(X gallons)(8.34 lb/gal)(0.01) = 5 lbs, then solve for X gal:
(X gallons) = 5 lbs = 60 gallons
(8.34)(0.01)
We the get the pounds formula:
(60 gallons)(8.34 lb/gal)(0.01) = 5 lbs
Setup for 5% solids:
(X gallons)(8.34 lb/gal)(0.05) = 5 lbs, solve for X gallons, get:
(12 gallons)(8.34 lb/gal)(0.05) = 5 lbs
Setup for 15% solids:
(X gallons)(8.34 lbs/gal)(0.15) = 5 lbs, solve for X gallons,
get:
(4 gallons)(8.34 lb/gal)(0.15) = 5 lbs
Setup for 50% solids:
(X gallons)(8.34 lb/gal)(0.5)= 5 lbs, solve for X gallons, get:
(1.12 gallons)(8.34 lb/gal)(0.5)= 5 lbs
We can bundle up all of this information into a table to see the "overview" more clearly. Here is our summary table:
| Gallons of sludge |
|
|
|
|
| Weight of solids, pounds |
|
|
|
|
| Percent solids |
|
|
|
|
| Pounds of water removed |
|
|
|
|
| Gallons of water removed |
|
|
|
|
If we make the cost to haul the biosolids to our reuse facility, at $5 per cubic yard, we can affix a cost to various solids concentration points.The following graph will illustrate this.
This graph really tells it all. We get some real value and benefit getting to 30% solids. Above 30% further concentration of solids is less dramatic in volume and cost savings to transport, etc.
From these examples, we find that as we concentrate and thickening our solids, we are able to realize a reduction in the number of gallons to process, lower-costs to treat in downstream process, and lower costs transport.
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