Wastewater containing 280 mg/L BOD, 380 mg/L as CaCO3
bicarbonate with a pH of 7.9 is being pumped to a small treatment
works 250 m away.
It is pumped at a rate of 450 m3/h via a 350 mm
diameter pipe made of ductile steel. The pipe is fitted with 4
close-radius 90° bends, 2 long radius 45° bends, and with a
sharp-edged entrance into an equalization tank, from a bell-mouth
entrance at the extraction
point.
The equalization tank is 22.5 m deep and 18 m in diameter, and
feeds a biological process tank which has the same aspect ratio
(height/diameter) as the EQ tank, but 45% of the residence
time.
The biological process generates a waste flow of 3% of the feed
flow with a solids concentration 15% higher than the bio-process
MLSS concentration of 3 g/L.
The biological process sludge MLVSS to MLSS ratio is
0.8
The treated water then passes through 15 cylindrical sand
filters operating at a pressure of 0.17 bar for the clean filter,
the mean sand grain size being 700 μm, and the bed porosity 32%
with an aspect ratio of 0.8.
The filtered water then passes through 2 stages of reverse
osmosis modules, each providing a conversion of
45%.
Finally, the water passes through a degasser to strip the
CO2 from the water using air containing 0.2%
CO2 flowing at 35 kg/min. The offgas produced contains
5% CO2.
The average ambient water temperature is 21°C.
(head loss = 32.16 m, residence time = 12.72 hrs, volume of tank
= 2576 m3, depth of biological tank = 17.24 m,
diameter = 14 m, and F:M ratio = 0.016/hr)
Question
1) How much permeate is produced by the RO array?
2) What is the dissolved CO2 concentration in the
product stripped water? (Assume that the feedwater to the
stripper contains all carbonate converted to the
CO2 form)
Draw the complete flowsheet including, where possible, the
stream flow rates and suspended solids concentrations.
