Page 268 - Membranes for Industrial Wastewater Recovery and Re-Use

Case studies 2 3 7

additional benefit is a reduction in operating cost of the demineralising plant
through a reduced number of regenerations. The saving predicted when the
plant is at full capacity is AUD$100 000 ($53 520) generating a total annual
savingofAUD$1200000 ($642 240).
The fixed cost of supply from the deed of agreement is a crucial aspect of the
economics as it generates significant cost reductions over the period of the deed.
The expected payback period on current numbers is between 6 and 7 years
enabling 8-9 years of annual saving of around AUD$1200 000 ($642 240) as
profit. The total saving over the period of the deed equates to AUD$ll 100 000
($5 940 720). The deed agreement was possible as Hunter Water Corporation
saved AUD$2 700 000 ($1 445 040) in expenditure on disposal pipeline.

5.3 Doswell combined cycle power plant: zero liquid discharge
(USA)

5.3.1 Background
The Doswell combined cycle facility is a 660 MW power plant owned by Doswell
Limited Partnership and operated by Bechtel and is one of the largest
independent power plants in the USA. The plant is designed for dispatchable load
operation working mainly on weekdays during the winter and summer and
contains two parallel units which share a common water treatment plant. The
plant is designed to burn natural gas and has been in operation since July 199 1.
A major concern about the plant was potential limitations on water supply
and wastewater discharges issued by the local government of Hanover, Virginia.
To address these issues the company decided to minimise water use and recycle
wastewater. As part of this process a number of conservation and reuse measures
were implemented at the site:

0 Air-cooled condensers were installed instead of the typical wet cooling
towers to eliminate water losses through evaporation and dramatically
reduce wastewater production.
0 Dry hybrid burners were installed that limited NO, formation without the
need for water or steam injection.
0 Potable water demand was reduced to a minimum by utilising sewage
effluent from the local wastewater treatment facility.
0 Any wastewater generated at the site was recycled through a zero liquid
discharge (ZLD) facility.

A key unit process of ZLD is vapour compression evaporation which can
recover about 95% of the waste stream as distillate. The remaining flow is then
converted into solids in a crystalliser/dewatering device. However, the process is
very expensive when flows are high and/or low in dissolved solids. Consequently,
the company investigated the possibility of pre-concentrating the flow with