Characterizing the impact of geometry on the performance of inline-coagulation pretreatment systems with helical flocculators

Abstract

Inline-coagulation (C-IN), where coagulant is added to the feed without solids separation prior to filtration, is a widely applied pretreatment strategy for low-pressure membrane fouling mitigation. At bench-scale, simulating C-IN has proven difficult due to the low flowrates involved. One promising approach to achieve continuous-flow operation in these small-scale systems is the helically coiled tube flocculator (HCTF), a coiled section of pipe. However, the impact of changing helical geometry on the properties of flocs formed by HCTFs, which can significantly affect membrane performance, has yet to be quantitatively characterized. Moreover, there are currently no widely accepted design guidelines for HCTFs. Accordingly, the current study uses a continuous-flow bench-scale system and a 2³ full-factorial experimental design approach to investigate the impact of HCTF geometry on the properties of flocs produced. The results showed that the floc size and concentration were most significantly impacted by the pipe length (i.e., retention time), while the floc fractal dimension demonstrated a relative insensitivity to helical geometry for all tested configurations. The Reynolds number was also shown to exert a critical impact on the properties of flocs, likely the result of a trade-off between mixing strength and retention time. When compared to a batch-reactor, the tested HCTF configurations produced smaller sized flocs in greater concentrations, while relative to a commercial inline static mixer, HCTFs yielded larger sized flocs in lower concentrations. The size of the flocs formed indicate that HCTFs are suitable for continuous-flow C-IN, with pipe length and system hydrodynamics being the most important parameters in their design. Additionally, the choice of mixer type significantly impacts floc properties, which are expected to alter the performance of downstream unit operations.