Surface water ultrafiltration pre-treatment through in-line coagulation and shallow bed media filtration

Date
2017-03
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH SUMMARY: High concentrations of organic and microbial matter in surface water cannot always be removed sufficiently through conventional water treatment methods. Ultrafiltration (UF) membranes are preferred for this purpose, as the low feed pressures required are cost effective, and a smaller footprint area is required compared to a conventional plant. UF membranes effectively clarify and disinfect surface water. One notable hindrance to the use of UF for surface water treatment is the tendency of the membranes to foul. Particularly with the first annual rains, very high organic and solids loadings enter rivers. UF membranes can foul rapidly under these conditions. Therefore, this project investigated the use of in-line coagulation and integrated shallow-bed media filtration as lowcost pre-treatment prior to UF. The media filter and UF were filtered and backwashed at the same intervals, using UF filtrate to simultaneously backwash both units. A shallow media bed with a depth of 250 mm promoted fractional particle retention and floc modification, by promoting contact between destabilised coagulated particles. The media filter, which acted as pre-filter (PF), was characterised for low turbidity river water of 12 NTU, represented by a prepared (synthetic) suspension of 20 mg/L humic acids (HA) and 30 mg/L bentonite. Design and operating parameters were chosen such that the PF could produce filtrate with turbidity below 1 NTU as feed to the UF, for 40-60 minutes. Backwashing the combined system at 4 PF displacement volumes (for 2 minutes) effectively restored the retention capacities of and initial pressure drops over both filters. The system was operated at an average downflow rate of 10 m/h through the PF and a flux of 46 LMH through the membrane. Keeping the parameters selected during characterisation fixed, the performance of the combined system was tested with higher turbidity synthetic suspensions. The improvement brought about to membrane performance by the PF was of interest. For the low (12 NTU) and high (31 NTU) turbidity suspensions, reversible fouling occurred on the membrane, with a large percentage of the loading removed by the PF. No discernible irreversible membrane fouling took place. For low turbidity water, pre-treatment completely reduced TMP over the membrane, while at high turbidity water (25 mg/L HA, 100 mg/L bentonite), TMP was reduced by 70%. For the latter, breakthrough occurred after 15 minutes for 10 m/h PF filtration rather than after 50 minutes. With a very high solids feed loading (400 mg/L bentonite) at 8 m/h PF downflow, the filtration duration had to be shortened to 50 min for effective PF backwashing. The PF could still reduce TMP by 60%, and allowed a more gradual turbidity filtrate profile to be filtered by the UF. High organics loading of 70 mg/L HA did not present the same problem, and were effectively coagulated in such a manner that 99+% of TMP could be reduced through pre-filtration. On-site testing confirmed lab findings that most of the turbidity load could be removed by the PF, indicating that overall system recovery can be increased by prolonging filtration duration. However, this work unfortunately took place at a time when the feed water was rather clean (6 NTU). On-site test work needs to be conducted with high turbidity feed to evaluate the performance of the unit under such adverse conditions.
AFRIKAANS OPSOMMING: Toenemend hoër konsentrasies van organiese en mikrobiese spesies in oppervlakwater kan nie altyd deur konvensionele watersuiweringstegnologieë tot ‘n veilige mate verwyder word nie. Ultrafiltrasie (UF) membrane word verkies vir die behandeling van hierdie waterbron, as gevolg van die koste-effektiewe lae druk benodig om die stelsel te bedryf, asook die kleiner aanlegarea in vergelyking met konvensionele aanlegte. UF verwyder troebelheid asook patogene in water. Die grootste hindernis tot die meer wydverspreide gebruik van UF in oppervlakwaterbehandeling, is die neiging van membrane om bevuil te word. Veral met die eerste jaarlikse reënval word hoë organiese en siltladings in riviere vervoer, wat membrane kan beskadig. Om hierdie rede was die moontlikheid ondersoek om koagulasie en ‘n geïntegreerde vlak bed media filter te gebruik as UF voorbehandeling. Die media filter en UF word terselfdertyd bedryf en teruggewas. UF permeaat word as waswater vir beide filters gebruik. ‘n Vlak media bed met ‘n diepte van 250 mm het turbiditeit gedeeltelik verwyder asook flokkulasie van destabiliseerde partikels aangehits. Die media filter, wat as prefilter (PF) gedien het, was vir sintetiese water (bestaande uit 20 mg/L humiese sure (HA) en 30 mg/L bentoniet) met ‘n turbiditeit van 12 NTU gekarakteriseer. Ontwerps- en bedryfsparameters was so gekies dat die PF vir 40-60 minute filtraat kon lewer met ‘n troebelheid onder 1 NTU. Deur die stelsel na deurbraak terug te was vir 4 verplasingsvolumes (2 minute) was die PF se filterkapasiteit en aanvanklike drukval oor beide filters effektief herstel. Die stelsel was vir gemiddeld 10 m/h vloei deur die PF en 46 LMH flux deur die membraan bedryf. Karakterisasieparameters is konstant gehou vir behandeling van sintetiese waters met hoër turbiditeite. Die verbetering wat die PF aangebring het vir UF waterbehandeling was van belang. Vir water met lae (12 NTU) en hoë (31 NTU) troebelheid was bevuiling omkeerbaar, en die PF het ‘n groot lading van die membraan verwyder. Vir lae turbiditeit water het die PF TMP met 99+% verlaag. Selfs vir die hoë turbiditeit water (opgemaak uit 25 mg/L HA, 100 mg/L bentoniet) het die PF TMP met 71% verlaag. Vir laasgenoemde sintetiese oplossing het deurbraak na 15 minute plaasgevind eerder as die 50 minute waarvoor die laer turbiditeit water filtreer kon word. Vir vloedtoestande met hoë bentonietlading (400 mg/L) moes die filtrasiesiklus verkort word na 50 minute vir 8 m/h PF filtrasie. TMP is met 60% verlaag, met ‘n meer geleidelike PF filtraat turbiditeitskurwe wat deur die UF filtreer is. Vir ‘n hoë organiese lading (70 mg/L HA) kon die PF die TMP met 100% verlaag. Die opstelling is op ‘n rivierwatersuiweringsaanleg getoets ten tye van relatiewe lae rivierwater troebelheid (6 NTU). Toetswerk het aangedui dat die meeste van die water troebelheid en feitlik 99+% van TMP deur die PF verwyder was vir kontinue filtrasie. Dit sal moontlik wees om onder sulke omstandighede die filtrasietydperk te verleng om terugwasverlies te verminder. Die opstelling moet op ‘n aanleg getoets word op ‘n tyd met hoër troebelheid water om die werking van die gekombineerde stelsel onder sulke omstandighede te toets.
Description
Thesis (MEng)--Stellenbosch University, 2017.
Keywords
Ultrafiltration, Water -- Purification -- Coagulation, Water -- Purification -- Membrane filtration, Water -- Purification -- Shallow media bed filtration
Citation