Question - You are to treat a wastewater that is contaminated with 100 mg/I of SO₄^2- as S. You will do the treatment biologically by sulfate reduction, in which SO₄^2- -S is reduced to H₂S-S, which is stripped to a gas phase. You do not need to be concerned with the stripping. The electron donor will be acetate (CH₃COO^-), which you will need to add, say, in the form of vinegar. You know the following about the wastewater: flow rate = 1,000 m³/d, sulfate = 100 mg/l as S, volatile suspended solids (X_v⁰) = 0.

From previous tests and theory, you have estimated the following parameters for the rate-limiting substrate, which will be acetate (expressed as COD):

F_s⁰ = 0.08

q^{^} = 8.6 g COD/g VSS_a-d

b = 0.04/d

K = 10 mg COD/l

f_d = 0.8

N-source = NH₄⁺-N

You also have selected these design criteria:

• Use a solids-recycle system with a CSTR reactor
• Have θ_x = 10 d and θ = 1 d
• Have an effluent sulfate concentration of 1 mg S/l

You must provide the following crucial design information.

(a) Compute the effluent concentration of acetate (in mg/l of COD).

(b) Compute (using stoichiometric principles) the input concentration of acetate (in mg/l as COD) needed to take the effluent sulfate concentration from 100 to 1 mg S/l. [Note that the formula weight of S is 32 g.]

(c) Compute the concentrations (in mg VSS/l) of active, inert, and total volatile suspended solids.

(d) Compute the waste-sludge mass flow (in kg VSS/d) and volumetric flow rate (in m³/d) if the effluent VSS is 5 mg/l and the recycle VSS is 5,000 mg/l.

(e) Compute the recycle ratio R.

(f) Based on the (normal) multisubstrate approach, determine the effluent concentration of SMP.

(g) Do you see any problems with the design? If yes, explain the cause any problems and ways to overcome them.