Ragging phenomenon characterisation and impact in a full-scale MBR

Abstract

Although there are few studies about clogging phenomenon in the peer-reviewed literature, it is considered one of the main operational challenges by membrane bioreactor (MBR) practitioners. This study presents data from the performance of a full-scale MBR affected by clogging, and ragging in particular. An evaluation of the efficiencies of different applied cleaning methods revealed the acid recovery cleaning to be more efficient than the basic recovery cleanings, although all maintenance cleanings were largely ineffective in recovering membrane permeability. Only declogging cleaning through the manual removal of the accumulated solids was found to be efficient, indicating that such solids were substantially unremoved by chemical cleaning. Moreover, reclogging following manual cleaning demonstrated a propensity for rapid clogging - within a period of 10 days over which the permeability returned to 68 and 88% of the pre-cleaned state. The analysis of the feedwater indicated suspended textile fibres (>70% cotton) to be present at a concentration of more than 40 mg·L(-1), ∼90% being smaller than 1 mm (0.06-0.4 mm). These small lengths of filaments evidently pass through pre-treatment and are retained on the membrane surface, forming 'rags' within the membrane module, notwithstanding the routine high quality of sludge reflected in the capillary suction time and filterability measurements. Pre-treatment improvement, manual cleaning and permeate flux reduction are the only options to minimise ragging impact over MBR performance.

What have we learned from worldwide experiences on the management and treatment of hospital effluent? — An overview and a discussion on perspectives

“…As reported in the investigation by Verlicchi et al (2008), the raw HWW may contain rags, filaments, pieces of cardboard that can adversely interfere with moving parts within the WWTPs or clog membranes and thus they have to be efficiently removed at the start of the treatment train. This is in agreement with suggestions by Gabarron et al (2013) which investigated different pretreatment processes to find the most adequate technology that would consistently contribute in minimizing the ragging impact over MBR performance.…”

Biological treatment and thickening with a hollow fibre membrane bioreactor

“…The observed impact of clogged solids on permeability of the subsequently cleaned membrane corroborates outcomes from a previous study (Buzatu et al, 2012), as well as field observations (Gabarrón et al, 2013), in that the deleterious impact of clogging on membrane permeability is retained even after declogging and cleaning. A repeat of the trial following declogging and a subsequent aggressive CEB at a very high hypochlorite concentration of (a) (b) (c)…”

“…As with almost all studies of permeability decline in MBRs, the few studies of membrane thickening of WAS have tended to be focused on characterisation of chemical foulants. However, it is broadly recognised by both the practitioner community (Stone and Livingston, 2008;Santos et al, 2010;Mason et al, 2010;) and, increasingly, by the academic community (Lebegue et al, 2009;Dvořák et al, 2011;Buzatu et al, 2012;Gabarrón et al, 2013) that clogging -the accumulation of solids within the membrane interstices (or spaces between the fibres) -represents as significant challenge to sustainable MBR operation as membrane surface fouling. Since clogging propensity would be expected to be greater at elevated sludge concentrations, it is of obvious importance to assess the relative significance of fouling and clogging in such cases.…”

Activated Sludge – 100 Years and Counting

“…for an HF membrane, and less frequently for FS membranes. Whilst the latter has conventionally been attributed to poor pretreatment, it appears that rags form in situ from the agglomeration of small lengths of textile fibres (Stefanski et al, 2011;Gabarrón et al, 2013). Since the ori ginal virgin membrane permeability is never recovered once a membrane is fouled through normal operation, there remains a residual resistance which can be defined as 'irrecoverable fouling'.…”