ADVANCED SEPARATION TECHNOLOGY APPLICATION FOR NOM REMOVAL FROM A FRESHWATER SUPPLY

Transcription

1 Costa Mesa, July 27, , July 29, 2011 ADVANCED SEPARATION TECHNOLOGY APPLICATION FOR NOM REMOVAL FROM A FRESHWATER SUPPLY Andrea G. Capodaglio,, Arianna Callegari and Philippe Sauvignet 650th Anniversary

2 Surface water bodies contain diverse types of aquatic natural organic matter NOM. NOM can be microbially derived (autochthonous), resulting from processes such as leachate and extracellular release of algae and bacteria, and terrestrially derived (allochthonous( allochthonous), originating from decomposition and leaching of plant and soil organic matter. NOM is a complex heterogeneous mixture of organic compounds, consisting of aromatic, aliphatic, phenolic,, and quinonic structures with varying molecular sizes and properties. This made its structural and functional characterization extremely difficult. One common approach for its characterization is to divide the mixture into the hydrophilic ic and hydrophobic fractions.

3 NOM classification (Leenheer and Crouè,, 2003).

4 Regardless of the source, NOM should be removed from drinking water, since it may: affect organolectic properties of water (color, taste and odor); react with disinfectants reducing their disinfection power, producing undesirable by-products (DBP( DBP s), influencing disinfectant demand; affect process design, operation and maintenance; affect stability and removal of inorganic particles; control coagulation conditions and performance, and influencing dosage; affect corrosion processes; affect biostability and biological regrowth in distribution systems; forms complexes with, and increases mobility of, chemical substances found in nature; foul membranes; reduce adsorption capacity of granular or powdered activated carbon (GAC/PAC) by pore blocking; compete with taste and odor compounds for adsorption sites in GAC/PAC.

5 STATE-OF OF-THE-ART TECHNOLOGIES FOR NOM REMOVAL Coagulation is probably the most commonly used method for NOM removal. The conventional process incorporates several physicochemical processes including rapid mixing, slow mixing (flocculation), sedimentation, filtration, and disinfection. Coagulation reactions take place almost instantaneously in the rapid mix stage of the water treatment process and continue until the water is filtered. The effectiveness of coagulation affects the efficiency of the subsequent sedimentation and filtration processes.

6 Effective coagulation is achieved through addition of charged (or other destabilizing) species into the water source; the two most commonly used in practice being: aluminum sulfate (Al 2 (SO 4 ) 3 14H 2 O), and ferric chloride (FeCl 3 6H 2 O). Aluminum sulfate, commonly referred to as alum, is the most widely used coagulant in water treatment processes. Several factors influence the efficiency and effectiveness of coagulation by metal salts. These factors include coagulant dose, ph, alkalinity, temperature, and ions present in solution. Ambient ph is critical in maximizing NOM removal.

7 Activated carbon filtration The adsorption behaviour is difficult to understand due to its heterogeneous nature. Studies show that NOM adsorption is controlled by the relationship ip between the molecular size distribution of NOM and the pore size distribution of the carbon. The lifetime of GAC filters can be expanded by carbon reactivation, however, thermal reactivation can lead to an enlargement of the macropores in the carbon because of burn-off effects, and increase the removal of the high molecular weight matter and decrease that of low molecular weight.

8 Direct activated carbon adsorption is not recommended since the sorption capacity is quickly reduced by pore blocking caused by the large humic substance molecules. Coagulation prior to the granular activated carbon (GAC) filtration can remove particles that might clog the GAC filter. Coagulation also removes NOM, which reduces the loading on the GAC filters. GAC adsorption can be applied following water preoxidation by ClO 2 ; this may lead to the formation of organic by-products due to interactions between GAC, NOM and ClO 2. ClO 2 may cause a break-up of the large molecules, and alter the molecular size distribution of NOM towards smaller molecules. On the contrary, oxidation with small doses of ClO 2 can increase the molar masses of some organic matter molecules.

9 Membrane filtration Nanofiltration (NF) technology has proved to be a successful alternative process for drinking water treatment, due to its superior removal of disinfection by-product precursors, minimal use of chemicals, reduction in sludge production, and potential for use in compact systems. The cost of the process is at the moment higher than those of coagulation and GAC adsorption; however the rate of decrease in costs over the past years has been greater than those associated to other treatments.

10 Studies carried out in Norway showed that the criteria for success ss in operating NF-plant for NOM-removal consist of the: use of low flux (<20 l/m 2 h) low recovery (<70%) use of cellulose acetate membranes (to avoid adsorption fouling) adoption of proper cleaning procedures (daily with a diluted solution, combined with a more comprehensive chemical cleaning once or twice a year)

11 Advanced oxidation processes An alternative group of technologies that can be used to remove NOM and minimize the formation of DBPs,, are advanced oxidation processes (AOPs( AOPs). AOPs are defined as near-ambient ambient temperature processes that involve the generation of highly reactive radical intermediates, especially hydroxyl radicals. These radical are extremely reactive and capable of oxidizing some of the NOM present in raw waters. The most common process to generate OH is through the use of combined catalytic oxidant such as ozone-ultraviolet (O 3 -UV) and hydrogen peroxide ultraviolet (H 2 O 2 -UV). Although all of the above processes can produce OH, the O 3 -UV process provides the maximum yield of OH per oxidant

12 A NEW ADVANCED SEPARATION TECHNOLOGY FOR NOM REMOVAL Ballasted flocculation is a physical-chemical chemical separation process that employs a high density additive (usually sand) to promote the t formation of a heavier floc,, which then settles more rapidly than in the traditional process. In this instance, a patented version of ballasted flocculation Actiflo Turbo,, developed by Veolia Eau, was modified by means of activated carbon (PAC) injection into the pre-contact chamber, in order to verify its NOM removal capacity in a practical situation.

13 The system, now denominated Actiflo Carb Carb,, is equipped with a pre-contact tank where the incoming raw water flow is quickly mixed to the PAC (both fresh and recirculated), allowing NOM adsorption (contact time about 4 min). Subsequent addition of coagulants in a second smaller chamber at slow stirring, of polymers and of Actisand (a patented, calibrated type of sand) in a third flocculation basin, allows the generation of heavy flocs that are subjected to ballasted flocculation and sedimentation within a fourth basin equipped with tube settlers. The settled sludge consisting of used PAC and Actisand is then recirculated to a hydrocyclone where the two components are recovered, to be reused in the process.

14 Actiflo Carb process flowsheet.

15 An Actiflo Carb pilot plant, with the capacity of 50 m 3 /h was installed at the water treatment plant in Brittany (France), where supply water from the local lake, rich in NOM contents, is treated to drinking water standards. s The pilot is positioned off-line after the existing flotation tank and will substitute the existing oxidation/sand filtration/oxidation/gac process train.

16 In order to optimize the performance of the Actiflo Carb plant, jar tests have been carried out to determine the type and ideal dosage of reagents that must be added during the process. The tests also allow to determine the sequence with which the reactives (FeCl 3, aluminum, polymers and PAC) should be added and their dosages; other variables included operational ph, Actisand composition (granulometry( granulometry) ) and contact times. In the case of PAC, one additional parameter was the ratio between new and recirculated PAC. A large number of possible combinations among these factors were investigated.

17 Jar tests picture.

18 Optimal dosages for the coagulant.

19 Variation of turbidity and absorbance with varying types and dosages of microsand.

20 Effect of the organic coagulant polymer on turbidity and UV absorbance.

21 Influence of PAC contact time on effluent turbidity and UV absorbance.

22 NOM removal was determined in term of COD and turbidity removal efficiencies using a UV turbidimeter,, which was preventively proved to carry very high correlation with traditional measurements methods. The results obtained show that COD removal is somehow proportional to PAC dosages, optimal PAC dosages lie generally below 25 mg/l, as higher feed rates do not achieve a correspondingly higher COD removal.

23 Relationship between PAC dosage and COD removal.

24 Turbidity is an important control parameter representing correct operation of the flocculation process. As particles with diameter between 1 and 10 µm m (mostly colloidal) do not settle well by gravitational forces alone, they constitute e the primary turbidity cause in the raw water. Surface properties, not mass, is the factor that keeps them in stable suspension, therefore consistent floc formation is a key requirement to achieve their removal. An optimal identification of reagent types and dosages allowed to t obtain consistent values of effluent turbidity well below 0.5 NTU even with influent values > 4 NTU.

25 Influent and effluent turbidity during the study.

26 Conclusions The use of powered activated carbon to enhance drinking water treatment processes, can nowadays be considered an almost standard procedure. This study presents an innovative application where the activated carbon is added to the patented reactor Actiflo Turbo.. This unit is a clari-flocculator whose special performance is based on a patented design and to the use of a special calibrated type of sand that allows very fast settling of the flocs. Actiflo Carb Carb thus combines the benefits of ballasted clarification and of the adsorption capabilities of carbon. This unit, located downstream of a existing pretreatment, allowed significant additional removals of organic matter (as COD, > 50%) and turbidity (from 50% to 90%) compared with more traditional methods. The key to the success of this process is its correct setup with the identification of optimal types and dosages of reagents.