1. In wastewater treatment, sludge is obtained from the clarifiers in the primary and secondary
treatment processes. The sludge must be subjected to processes that will “stabilize” it prior
to its disposal. This means that much of the organic material in sludge must be biodegraded
and the concentrations of potentially pathogenic organisms must be significantly reduced.
One of the processes used for this purpose is anaerobic digestion. The sludge is held in a
closed tank at a temperature of approximately 35 °C for 15-25 days, and anaerobic microbes
(microbes that grow in the absence of oxygen) convert much of the organic matter into CO2
and methane (CH). Since these anaerobic organisms are highly pH-sensitive, the alkalinity
of the digester contents is important and often monitored. Due to the high CO2 concentration
in the digester head space (approximately 30% of the total gas), the total Co2/HCO3’/Co3=
concentration is rather high: Cr : [HZCOS‘]+[HCO3‘]+[CO3=] : 0.021 M.
Graph alkalinity (in M) vs. pH from pH 3 to 12. How does alkalinity vary with pH in such a
digester? Explain the shape of the curve (specifically where the slope changes). If the pH in
the digester is 6.6, what is the concentration of HCO3’? Because we have not discussed air-
water partitioning, ignore the effect of CO2 in the gas phase. (2 pts)
2. What is the pH of a 1.0 mM solution of (a) NaOCl, (b) Ca(HCO3)2, (c) Na2Co3? Solve them
graphically. (3 pts)
3. (a) A wastewater contains 10 mM ammonium acetate (NH4Ac, Ac- : acetate). If NH4Ac is
the only compound in the wastewater that has acid-base properties, and if the plgs of NH,+
and acetic acid (HAc) are 9.25 and 4.75 , respectively, what is the pH of the wastewater? We
did this problem numerically. Now solve it graphically (using a pC-pH diagram). (1 pt)
(b) IfI acidify the solution and lower its pH to 5.0, what would be the concentrations of HAc,
Ac”, NH4“, and NH3? Obtain these concentrations from your pC-pH diagram. (1 pt)
4. Ammonium dihydrogen phosphate (NH4H2Po4) is a fertilizer that provides both nitrogen (N,
as NH,*) and phosphorous (P, as H2Po4‘) as plant nutrients. These nutrients are considered
pollutants in natural waters as they may cause excessive algal growth and eutrophication.
(a) Write charge balance and proton condition equations for NI-LHQPO4 in water. (1 pt)
(b) What is the pH of a 1.0 mM NH4H2PO4 solution? The pKa of NH,+ is 9.3 and the 3 pK,s
of H3PO4 are 2.1 , 7.1 and 12.3. Solve the problem graphically. (1 pt)
(c) What are the major nitrogen and phosphorous species at that pH? (1 pt)