Abstract
Abstract A pulsating floc blanket clarifier (PFBC) employing cyclic contractions and rarefactions to a bed of densely concentrated suspension of flocculated particles in a fluidized state, was compared with conventional clariflocculator (CC) at pilot scale (8,000 L/day) in continuous mode of operation. For influent turbidity varied from 2 to 20 NTU, coagulation-flocculation behavior exhibited under the two fundamentally different treatment processes with PACl influenced inter-related performance parameters. The residual turbidity was found to be lower by 74%, flocs and fine colloids in suspension larger by 73 and 75% respectively, and the total and dissolved residual aluminum lower by 50 and 49% respectively on average for PFBC compared to CC. Particulate form comprised major fraction (≈72%) of total residual aluminum for both. PFBC abetted formation of a more consolidated floc structure, which rendered the shape, size and morphology such that the settling velocity was 50% to 410% higher than that of the CC flocs. Reaction-limited aggregation (RLA) process and inter-particle bridging were dominant and the resulting floc structure and its formation mechanism have been presented.
Highlights
Physico-chemical unit processes like coagulation, flocculation and clarification have been carried out in separate basins in series
The results showed a weak negative correlation between influent turbidity and residual aluminum, but the importance of higher influent turbidity was less for pulsating floc blanket clarifier (PFBC) compared to CC since the blanket provides that environment
Comparative performance assessment of pulsating floc blanket clarifier (PFBC) and conventional clariflocculator (CC) along with examination of flocs formed in the two processes with poly aluminum chloride (PACl) was carried out
Summary
Physico-chemical unit processes like coagulation, flocculation and clarification have been carried out in separate basins in series. Apart from the space requirements, conventional clariflocculators (CCs) employ mechanical components at various stages, adding to complexity as well as running and maintenance costs (Dhabadgaonkar 2008). CCs have been struggling to meet the drinking water regulatory requirements, especially in low turbidity (,10 NTU) and low alkalinity (,50 mg/L as CaCO3) conditions as their dominant mechanism for particle removal; namely, sweep floc action suffers adversely (Cheng et al 2008; Edzwald 2020; Srivastava et al 2020). A zone of densely concentrated suspension of flocculated particles in fluidized state, or in other words, a floc blanket, can be advantageously used in water treatment, since the condition of low influent turbidity is compensated by it. For the purpose, Srivastava et al (2020) had described the pilot scale experimental setup for comparatively evaluating the upflow pulsating floc blanket technology and conventional upflow clariflocculation
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