Chemical Unit Process

Chemical Coagulation  Flocculation  Clarification Chemical Precipitation Chemical oxidation Scale control
Chemical Coagulation Convert undesirable soluble substances into Chemical oxidation in WWT uses oxidizing agents Scalants – CaCO3  due to high pH/alkalinity
Suspended particle gravity removal insoluble precipitate which can be easily & rapidly to remove recalcitrant organic matters, reduce
Colloidal particle gravity removal removed. metals & disinfection. Required for nanofiltration & reverse osmosis (RO)
 Colloidal stability: to control formation of scale.
-ve surface charge Repel, stabilized by Applications: Application of chemical oxidation in WWT
electrostatic repulsion force  Don’t coagulate - Improve treatability of non-biodegradable Scales formed
Reduce in Membrane
organic compounds. in treatment failure
Enhanced removal of TSS (Sweep-floc) performance
- Eliminate inhibitory effects of certain module
Determined by surface charge density  Zeta Sludge generated:
Potential  Large +/- zp = dispersion stability organic/inorganic compounds to microbial
Hence, important to
growth.
To avoid formation adjust characteristic
Coagulants  Destabilize colloids by reducing - Reduce/Eliminate toxicity of certain of the effluent to be
Removal of soluble Phosphorus of CaCO3 & sulfate
repulsion forces btwn colloids  stick tgt when in organic/inorganic compounds to microbial treated
Use metal salts & polymer/lime. growth & aquatic flora.
contact
Lime: Scaling control
Lime reacts with natural alkalinity & form carbon strategies:
Coagulants: Example:
precipitates, enhances SS removal. (CaCO3 scale control achieved with these methods)
Alum Organic Simple end - Acidify to reduce pH & alkalinity.
Intermediate
 pH 5.2-8.8 produce solid & effective coagulant pH>10, excess calcium ion reacts with phosphate. molecules oxygenated products - Reduce calcium conc. by ion exchange or lime
 Weak acid, pH of WW, add lime when (COD) molecules (CO2, H2O) softening.
insufficient alkalinity - Lower the product recover rate.
Amount of lime Redox reaction
Ferrous Sulfate - Antiscalant scale inhibitor chemical) added to
- Depends on alkalinity of WW
 Need dissolve O2 , not used often increase apparent solubility of CaCO3 in the
- Independent on phosphate conc.
Flocculation concentrate stream.
Agglomeration of destabilized particles into a larger
size particle (flocs) which can be effectively removed Alum & iron (Trivalent ions):
Analysis of scaling potential:
by sedimentation and flotation. Precipitation is influenced by
Water is unstable if it deposits scales or corrodes
Alkalinity, pH, trace elements, ligands.
pipes.
Colloid enmeshment: Stabilization – correction of water by adjusting pH
Excess coagulant reacts with alkalinity to form Strategies for phosphorus removal: Half-reaction & alkalinity.
insoluble OH- which entraps colloidal particles P precipitation can occur at different locations.
(Sweep-floc coagulation) Pre-/Co-/Post- precipitation process
Langelier saturation index (used in water & WW)
Approx. tendency to develop CaCO3 (Alk)
Inter-particle bridging: LSI = pH - pHs
Synthetic polymer absorbs colloidal & coagulant Removal of Heavy metals LSI
particles & grow into a bigger & tougher floc. - Adjust pH to minimum metal solubility by Water is supersaturated with CaCO3. Scaling occurs.
adding lime or caustic Reaction Potential (Eo)
- Removed separately or co-precipitated with EReaction = EReducation – EOxidation Water is neutral. No scale form or remove.
Chemical Coagulation  Flocculation  Clarification Phosphate
- Minimal conc. in effluent depends on conc. & EReaction > 0  Reaction proceeds as written Water is undersaturated wrt CaCO3.
nature of organic matter and temperature EReaction < 0  Reaction proceeds in the opposite Water has tendency to remove existing scales.
- Need lab bench scale or pilot test direction from what’s written
Ryznar stability index, RSI (used in IWW)
Equilibrium constant (K) RSI = 2pHs – pH

Rapid mix Slow mix No mix