Question 12.9: WATER QUALITY ANALYSIS TO DECIDE CHEMICAL TYPES, DOSAGE RANG...

(a) Chemical Selection:
The raw water turbidity ranges from 25 to 200+ NTU, which requires 20 40 mg/L of alum, as Al_{2}(SO_{4})_{2}⋅14.3H_{2}O. Since it is the optimum jar test, alum is concluded to be the required coagulant. Alkalinity chemical should be fed only if natural alkalinity is insufficient. According to the stoichiometry equation shown in Example 11.3, one mole of alum Al_{2}(SO_{4})_{2}⋅14.3H_{2}O will react with 3 moles of alkalinity Ca(HCO_{3})_{2}  as  CaCO_{3}

Molecular weight of Al_{2}(SO_{4})_{2} ⋅ 14.3H_{2}O
= 2 × 27 + 3(36 + 4 × 16) + 14.3 × (2 + 16)
= 611.4

Molecular weight of 3CaCO_{3}
= 3(40 + 12 + 3 × 16)
= 300

The alkalinity requirement
= (20 to 40 mg∕L) × (300∕611.4)
= 9.81 to 19.62 mg∕L as CaCO_{3}

Since natural alkalinity in Springfield raw water ranges from 78 to 128 mg/L as CaCO_{3}, no extra alkalinity is needed.
After chemical coagulation, the treated water pH = 6.4. Therefore, pH adjustment chemical is required. Other required chemicals include, at least, disinfectants and corrosion control chemicals.

(b) Feeder Selection:
Alum is available in dry form and liquid form; therefore, dry alum can be fed with dry feeders, while the liquid alum can be fed with liquid feeders. Dry alum may be supplied in lumps, or in ground, rice or powdered form. Shipment of dry alum may be in 100-lb (45.4 kg) bags, in drums or in bulk truck quantities. Liquid alum is supplied as 40–50% solution delivered in minimum loads of 4,000 gal (15,140 L). The choice between dry or liquid alum use is dependent on factors such as water treatment plant size, storage space availability, feeding method, and economics. For large WTPs with design flow greater than 2.5 MGD (9.46 MLD), purchase of liquid alum is justified only when alum supplier is close enough to make differences due to negligible in transportation costs. For small WTPs with design flow 2.5 MGD (9.46 MLD) or below, liquid alum is used for the purpose of simplicity.

Since Springfield WTP’s design flow = 1.2 MGD (4.54 MLD), it is a small WTP. Assume liquid alum is available locally, the plant manager selects the liquid alum feed system using ground storage (shown in Fig. 12.4b).

From Example 12.1, a liquid alum with a sp. gr. of 1.3182 has a dry alum content of 5.12 lb/gal (614.15 g/L). Since the design flow = 1.2 MGD (4.54 MLD), and the alum dosage range is 20–40 mg/L as Al_{2}(SO_{4})_{2}⋅14.3H_{2}O, the liquid alum feeder must cover at least the range determined in below using the simplified material balance method:

(Q_{wtp})(C_{wtp}) = (Q_{lc})(C_{lc})
(4.54MLD)(20 to 40 mg∕L) = Q_{lc}(614,150 mg∕L)

Q_{lc} = 0.0001478 to 0.0002957 MLD
= 102.6 to 205.2 mL∕min

The actual liquid alum feeder should be wider than 102.6–205.2 mL/min. Besides, an identical spare feeder is needed.