Page 211 - Water and wastewater engineering

5-14 WATER AND WASTEWATER ENGINEERING

5-5 CHEMICAL COMPATIBILITY

It is not intended here to present an exhaustive list of incompatible chemicals but rather to
highlight combinations that may lead to violent reactions. Many others used in water and waste-
water require care. In particular, the design should provide sufficient piping and storage vessels
that the potential for residues from prior-use chemicals reacting with new-use chemicals is
minimized.
Because there is a tendency for water of crystallization from alum to slake lime, it is impera-
tive that mixture of these two chemicals be avoided. In a closed container, this combination may
lead to a violent explosion. For the same reason, ferric sulfate and lime should not be mixed
(Anderson, 2005). Mixtures of potassium permanganate and ferric chloride will form toxic chlo-
rine gas (Kawamura, 2000).
An abbreviated list of incompatible chemicals is presented in Table 5-4 . A rigorous search
for chemical combinations not shown in Table 5-4 is recommended as part of the design
process.

5-6 MATERIALS COMPATIBILITY

For very corrosive chemicals such as ferric salts, sodium hypochlorite, and sodium hydrox-
ide, nonmetallic materials are preferred. These include fiberglass-reinforced plastic (FRP)
and various forms of polyethylene (PE) such as high-density, cross-linked polyethylene
(HDXLPE).
Polyvinyl chloride (PVC), polypropylene, rubber-lined steel and type 316 stainless steel are
used for alum. In warm climates where the temperature of liquid alum may exceed 50 C, chlori-
nated polyvinyl chloride (CPVC) is recommended.
Lime and soda ash can be stored in concrete or mild steel silos.
Suitable materials for storage containers for other chemicals are listed in Appendix A. Rec-
ommended materials for piping are given in Table 5-5 .

TABLE 5-4
A short list of incompatible chemicals used in water and wastewater treatment
Chemicals Keep out of contact with: Remarks

Activated carbon Oxidizing agents such as chlorine, Potential for fire
hypochlorites, potassium permanganate,
sufuric acid
Alum Lime, milk of lime—Ca(OH) 2 Violent exothermic reaction
Ammonia Concentrated chlorine and Violent exothermic reaction
chlorine compounds
Ferric chloride Potassium permanganate Formation of chlorine gas
Fluorine compounds All chemicals Etches glass
Sulfuric acid Strong bases, light metal compounds Violent exothermic reaction
containing potassium and sodium