WATER & WASTEWATER

www.powermag.com POWER | March 201524

ery projects, specifically in zero-liquid

discharge, or near zero-liquid discharge sys-

tems, said Marchewka.

In addition to the FGD wastewater treat-

ment system Oasys installed at the Changx-

ing Power Plant, it has another FO system

already operating in China. That system has

the flexibility to be used for seawater de-

salination or for treating cooling tower blow-

down, depending on the plants needs.

Through a partnership with National

Oilwell Varco (NOV), Oasys technology

is being deployed in the oil and gas in-

dustry too (see this issues cover photo).

NOV says the system is suitable for on-

shore unconventional shale plays, and it

markets the solution as a means of treating

exploration and production wastewaters. It

touts that these streams can be converted

to freshwater quality, fully treated for re-

use in new drilling and completion fluids

or for surface discharge in remote areas

where disposal options have traditionally

been limited and expensive.

Oasys says it is the first company to de-

ploy an FO-based brine concentrator. The

company can also imagine using the technol-

ogy for things like brackish desalination and

other municipal applications.

One final advantage that really benefits

operators is the FO systems ability to handle

variation. Marchewka noted that the company

has learned from its experience in China that

the water chemistry from the FGD process

is quite variableseasons, load, and various

other operating parameters all factor in. Al-

though changes can be problematic for many

systems, because the FO system operates at

lower pressure and pulls the water across the

membrane with the draw solution, it is much

less prone to fouling and scaling, and it can

handle the challenge.

It actually gives operators a nice ben-

efit when dealing with fluctuations and

changes in water quality and water chem-

istry, says Marchewka.

Utilizing Treated Municipal WastewaterPower plants continue to face greater restric-

tions in the usage of water from traditional

sources, such as oceans, lakes, rivers, and

wells. In the U.S., regulations like 316(b) are

forcing facilities to consider alternatives to

business as usual. State-of-the-art technology

has made treated municipal wastewater gen-

erated by publicly owned treatment works

(POTW) an attractive source of cooling water

makeup for many power plants.

A study conducted at the University of

Pittsburgh, evaluating more than 400 existing

coal-fired power plants, revealed that 49.4%

of them could have sufficient cooling water

supplied by POTWs within a 10-mile radius

of their plant. If the radius were expanded to

25 miles, the percentage increased to 75.9%.

It also evaluated 110 proposed power plants

and found that 81% of those facilities could

meet their cooling water supply requirements

from POTWs within 10 miles of their pro-

posed locations. The 25-mile radius satisfied

all but three of the plants.

According to Kaveh Someah, vice presi-

dent of global energy for Ovivo USA, the

use of reclaimed water started decades ago

and is gaining in popularity. There are a

number of treatment technologies that must

be considered based on an individual plants

situation, but one of the more advanced

methods includes the use of a membrane

bioreactor (MBR).

An MBR is a wastewater treatment process

utilizing biological treatment alongside filtra-

tion all in one common tank. MBR systems

are considered the best available technology

for wastewater treatment and reuse applica-

tions, because they are reliable, space efficient,

and cost effective. Ovivoformerly known as

Eimco Water Technologiesworked with a

power plant in Texas to develop a solution that

uses an MBR system to provide makeup water

to the plants cooling pond.

The Membrane Bioreactor Treatment ProcessAt the Texas facility, the screen box design

handles course screening, allowing raw

wastewater to be pumped straight into a fine-

screening system to remove particles that

could potentially damage the membranes.

The screened influent enters the equalization

basin, which maintains flow forward up to

the peaking capacity of the membranes.

If sufficient hydraulic pressure is not

available, the plant is designed with an emer-

gency overflow to a basin located adjacent to

the equalization basin. Once plant flow and

level return to normal, any overflow can be

pumped back to the equalization basin for

feed forward.

From the equalization basin, screened and

equalized wastewater is pumped to the anox-

ic basin. The level in the anoxic basin varies,

depending on hydraulic loading conditions.

Control of the MBR plant is based on level in

the anoxic basin.

A programmable logic controller (PLC) re-

ceives a level input and varies the flow rate of

treated water to accommodate influent flow. It

also initiates an intermittent mode to preserve

biology, reduce power consumption during

low plant loading, and protect equipment.

A mixer in the anoxic basin operates con-

tinuously to mix the activated sludge with

incoming wastewater, maintaining a uniform

concentration of mixed liquor suspended sol-

ids. Pumps in the anoxic basin are used for

feeding forward and internal recycling.

4. A state-of-the-art wastewater treatment process. The submerged membrane bioreactor configuration relies on

course bubble aeration to produce mixing and

limit fouling. Courtesy: Ovivo USA

5. Waste not, want not. The Palo Verde Water Reclamation Facility can treat up to 90 million gallons of secondary effluent from the Phoenix metropolitan area and provides all of the

cooling water for the Palo Verde Nuclear Generating Station. Courtesy: Arizona Public Service