Process engineering: Wastewater treatment plant goes the distance

A mobile reverse osmosis (RO) system from USFilter effectively treated water at an abandoned fertilizer plant. This allowed system effluent to be discharged into Tampa Bay without environmental impact.

By Bill Perpich, Jr.

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Bicarbonates, mg/l

0.78

NA

Sulphates, mg/l

5,200

NA

Chlorides, mg/l

100

NA

Phosphates, mg/l

1,600

<0.5 mg/l as P

Nitrates, mg/l

0.26

NA

Flourides, mg/l

150

<5 mg/l as ion

pH

2.8

6.0-8.5

Silica, mg/l

200

NA

Iron, mg/l

5.6

NA

Manganese, mg/l

2.9

NA

TDS, mg/l

11,500

<50 mg/l

Turbidity, NTU

15

NA

TSS, mg/l

24

NA

Color, PCU

110

NA

BOD, mg/l

17

NA

TOC, mg/l

66

NA

TKN, mg/l

650

<2 mg/l

NH3-N, mg/l

600

< 1 mg/l

The objectives of Phase I were to demonstrate the feasibility of using RO and to reliably produce an effluent of 90 gpm within the limits shown in Table 1. Phase I was to be completed within three months or after 5.2 million gal. of treated water was produced, whichever came first. Production would then be ramped up to 450 gpm during Phase II based on the knowledge and experience gained during Phase I. Phase III was optional, and was proposed as a low-cost extension.

To meet the fast-track timetable, the team chose a mobile system. Within five weeks of the contract award, the system was started up. Phase I equipment included dual-media roughing filters, followed by multimedia polishing filters and two-pass RO (Figure 2).

The system operates without adjusting the low pH conditions in the feedwater. The RO section has two passes; the first operates at a very low pH (less than 3.0), and the second pass runs at close to neutral pH. The first-pass operates at low pH to prevent scaling due to silica, fluoride, sulfates and phosphates in the feed water, and to improve the rejection of ammonia. The main reason the second-pass runs at close to neutral pH is to improve the rejection of fluorides, silica, phosphate, organics and other weakly ionized compounds. The contractor added mobile ion exchange polishers to ensure consistent effluent quality within the stringent contract specifications. The raw water characteristics versus the system effluent quality during Phase I is shown in Table 2.

Table 2: Mobile system cleans up pond water

Parameter

Units

2002 Process Water Values

System Effluent

Contract Specifications

Color

PCU

70

NA

NA

Fluoride

Mg/L

170

<2

<5

Calcium

MG/l

591

<0.5

NA

Phosphorous as P

Mg/l

1,600

<0.2

<0.5

Ammonia

MG/l

700

<1.0

<0.9

pH

Units

2.85

6.0-8.5

6.0-8.5

Silica

Mg/l

210

<0.5

NA

Sulfate

Mg/l

4,600

<1.0

NA

Conductivity

μs/cm

10,500

<25

<50

Total Nitrogen

Mg/l

730

<1.0

<2.0

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