Page 116 - Membranes for Industrial Wastewater Recovery and Re-Use

lndustrial waters 95

concentration. Treatment requirements depend on discharge requirements or
incentives for reuse of the water. Typically discharge standards are based on pH,
temperature, C-BODS, TSS, toxic substances, and dissolved solids.
Several approaches are used for treatment of water associated with
thermoelectric power generation depending on the source of the cooling and
boiler water, the type of cooling system, and the discharge requirements. The
characteristics of the water used for make-up dictate the need for supplemental
treatment. Only minimal levels of treatment are normally required for once-
through cooling systems. The main water quality objective is to prevent scaling,
corrosion or fouling. Because of the relatively short contact time and relatively
low temperatures, effective cooling can be accomplished using chemical
amendments to act as corrosion and scale inhibitors. One of the factors limiting
the use of chemical amendments for once-through systems is that the water is
typically discharged back to the environment. Thus, the type of chemicals that
are used must meet local water quality requirements for discharges (Piller and
Coupe, 2001).
Recirculating systems are more vulnerable to scaling, corrosion and fouling
due to accumulation of solutes. Thus, treatment objectives are targeted at
reducing the mineral, nutrient, and/or organic content of the make-up water.
Management practices for the blowdown water include discharge to a
wastewater reclamation facility, blending with other waste streams, or
treatment either to recover the water for reuse or to meet discharge requiremcnts
through the use of various biological and physicochemical processes (Table 3.5).
Historically, the predominant treatment systems have been chemical-based
treatments including lime softening and chemical precipitation using metal
salts. Recently, the use of ion exchange, membrane systems, and thermal
processes has become more cost-competitive. The new generation of treatment
technologies, largely membrane based, have the advantage of producing a high-
quality product water that can be reused for duties such as boiler feedwater
(Sections 5.1 and 5.2). Disposal of brines and waste streams arising from these
processes remains a challenge, however, and if zero liquid discharge is demanded
recourse is inevitably made to evaporation, sometimes preceded by
electrodialysis (Section 5.3).

3.1.10 Current reuse practices and opportunities
The extent to which reclaimed water is used for cooling water and boiler water
applications depends on several factors. Firstly, the reclaimed water source must
be within a practical distance from the industrial, ideally less than 50 km.
Secondly, economic and environmental factors should be favourable for the use
of reclaimed water. The key issues are the relative availability/scarcity of
alternative sources of cooling water. In addition to the need for water, when the
use of once-through cooling is limited by regulatory or discharge requirements
reclaimed water can become a viable alternative. To date, the use of reclaimed
water is fairly well accepted throughout the world, and some examples are
summarised in Table 3.6.