Patent Publication Number: US-2011062081-A1

Title: Process for Treating Waste From a Membrane Filtration Plant

Description:
The field of the invention is that of water treatment. More specifically, the invention relates to processes for treating water including at least one membrane filtration step. 
     The invention applies in particular, but not exclusively, to treatments for water intended to undergo a reverse osmosis or nanofiltration membrane treatment. 
     The invention preferably applies to water potabilization processes. 
     Water for human consumption is conventionally subjected to a nanofiltration or reverse osmosis filtration treatment in order to reduce the content of pesticides and other organic micropollutants therein that can be removed by membrane processes. 
     Nanofiltration also enables bivalent anions, such as sulfates, to be removed, and also enables the content of other salts, such as nitrates, for example, to be reduced. 
     Reverse osmosis uses membranes similar to those of nanofiltration, but with a greater separation power. It enables almost all organic and inorganic pollutants to be removed from the water. Reverse osmosis is used in particular in the production of water for human consumption. 
     In addition, it is conventional to subject the water to a pretreatment upstream of the reverse osmosis or nanofiltration membrane treatments, in which said pretreatment consists of a low-speed liquid-solid separation (for example, simple or lamellar settling and/or direct bi-layer filtration, and/or flotation). 
     A coagulation-flocculation treatment is also frequently performed. 
     One disadvantage of the membrane filtration techniques is that it produces waste called “concentrates”, representing 10% to 60% of the initial flow, and which are in most cases filled with phosphonates. 
     These phosphonates come from sequestering agents injected upstream of the membranes. These sequestering agents are intended to prevent the precipitation of salts on the membranes. They are entirely stopped by those thus concentrated at around 2 to 7 times in the membrane waste. 
     However, the authorities tend to limit or even prohibit phosphonate waste in rivers or the ocean. This problem appears in particular for waste from potable water production plants, some of which is likely to reach rivers or seawater. 
     It is therefore necessary to provide a technique to prevent such waste. 
     This is an objective of the invention. 
     More specifically, the invention is intended to propose a technique for removing undesirable species in filtration waste such as phosphonates, applied to a water treatment including a pretreatment and membrane filtration step. 
     The invention is also intended to propose such a technique that enables the operating costs to be reduced by comparison with the processes of the prior art. 
     The invention is also intended to provide a technique that provides optimized reclamation methods for excess sludge. 
     Another objective of the invention is to provide such a technique with a simple deign that is easy to implement. 
     The invention also enables waste to be treated in order to upgrade it by using it for cleaning industrial structures such as, for example, sand filters. 
     These objectives, as well as others, which will be described below, are achieved by the invention, which relates to a water treatment process including a first pretreatment step producing pretreated water and sludge, in which said pretreated water is then subjected to at least one membrane filtration step producing waste and a permeate, in which said permeate is routed to a potabilization system, characterized in that said first pretreatment step includes a first coagulation-flocculation step, and in that said waste from said membrane filtration step undergoes a treatment phase including at least one second coagulation-flocculation step followed by a sedimentation step producing sludge, in which said sedimentation step is preceded by at least one step of adsorption on at least a portion of said sludge resulting from the pretreatment and/or on a portion of said sludge coming from said sedimentation step, in which said adsorption step is intended to eliminate the phosphates contained in said waste resulting from said membrane filtration step, and said process produces treated waste and excess sludge. 
     It is noted that said first pretreatment step producing pretreated water and sludge will preferably include a so-called primary sedimentation step and/or filtration step on a filter including a filtration medium or on microfiltration or ultrafiltration (MF/UF) membranes, in which the sludge is in the latter two cases produced by back-washings of the filter or microfiltration or ultrafiltration (MF/UF) membranes. 
     As indicated above, the phosphonates come from the sequestering agents injected upstream of the membranes, concentrated at around 2 to 7 times thereon. 
     However, as the sequestering agents are chelating agents, they are easily adsorbed on clays, calcites or metal hydroxides, which compounds are classically present in sedimentation sludges. These hydroxides come from iron- or aluminum-based coagulants used in the coagulation step. 
     The adsorption capacity of the sludges is therefore used to remove the phosphonates of the membrane filtration concentrates. 
     In addition, the process according to the invention enables the amounts of coagulant to be reduced, and therefore the corresponding operating costs to be reduced. 
     Indeed, in the case of a conventional coagulation-flocculation of the concentrates, the amount of coagulant is two to three times higher than in the case of the process according to the invention. This is due to the fact that a portion of the phosphonates is adsorbed on the sludge as indicated above. The residual to be eliminated therefore involves a lower consumption of coagulant. 
     It is noted that the use of sludge for the adsorption step does not lead to significant additional costs, as this sludge is a byproduct of the process according to the invention. Recycling this sludge is therefore inexpensive. 
     The coagulant is preferably an iron or aluminum salt, and said sludge is iron and/or aluminum hydroxide sludge. 
     According to another feature, said second coagulation-flocculation step is performed in at least two successive phases, the first under rapid agitation and the second under slow agitation. 
     According to a preferred embodiment, said adsorption and sedimentation steps are preformed in the same structure, namely a sedimentation tank, preferably for 3 to 90 minutes and most preferably for around 15 minutes. Said adsorption step is performed if necessary under agitation. 
     According to another feature, the process includes a step of using the excess sludge in land farming. 
     In this case, the process includes at least one step of concentrating said excess sludge. 
     In this way, the concentration of phosphorous in the sludge is increased, thereby improving the capacity thereof to fertilize the farming soil. This enrichment of the sludges with phosphorous is a benefit for the agricultural upgrade thereof, as the phosphorous concentration of the sludges enables better fertilization of the farming soil. Moreover, the excess sludge obtained by the process according to the invention is rich in phosphonates. However, the phosphorous in the form of phosphonates is less accessible to plants than the phosphorous in the form of phosphate. The phosphorous degradation thereof will therefore be slower, and therefore more beneficial for the soil. 
    
    
     
       Other features and advantages of the invention will become clearer on reading the following description of a preferred embodiment of the invention, provided by way of an illustrative and non-limiting example, and the appended drawings in which: 
         FIG. 1  is a synoptic representation of a water treatment process according to the invention; 
         FIG. 2  is a graph of the elimination of phosphorus in membrane concentrates with different amounts of sludge. 
     
    
    
     In reference to  FIG. 1 , the example relates to a water treatment process for potabilization, which includes, according to the invention, a pretreatment step and at least one membrane filtration step, a step of removing the phosphonates present in the membrane filtration concentrates by adsorption on the sludges resulting from the pretreatment. 
     As shown in  FIG. 1 , the water to be treated undergoes a primary sedimentation step  1  preceded by a first coagulation/flocculation step, at the end of which a clarified water and a physicochemical sludge are obtained. 
     The clarified water is then subjected to a membrane filtration step  2 , by nanofiltration or reverse osmosis, at the end of which the permeate obtained is routed to a potable water production unit. 
     According to the invention, the membrane treatment concentrates are then subjected to a treatment phase including a second coagulation/flocculation step  3  and a so-called secondary sedimentation step  5 , at the end of which a treated concentrate and sedimentation sludge are obtained. 
     According to the invention, a phosphonate adsorption step  4  is inserted between the second coagulation/flocculation step  3  and the sedimentation step  5 . 
     This adsorption step is performed for 10 minutes, under agitation, with an agitation speed of 60 rpm, on the sludge resulting from the primary sedimentation step  1  and on a portion of the sludge resulting from the secondary sedimentation step  5 , in which the excess sludge coming from said step is thickened, then upgraded by land farming. 
     The second coagulation/flocculation step  3  is broken down into two phases: a first phase under rapid agitation at 250 rpm, then a second phase under slow agitation at 60 rpm. 
     The coagulant is inorganic, preferably FeCl 3 , with a concentration ranging from 1 to 200 mg/l. 
     The flocculent is of the 4190 SH Floerger type (registered trademark), with a concentration of between 0.05 and 1 ppm. 
     The duration of the sedimentation steps  1  and  5  is 15 minutes for each. 
     To show the efficacy of the process, primary sedimentation sludges coming from a potable water treatment plant and having different concentrations of suspended solids were placed in contact with the concentrates of the membrane filtration unit of said potable water treatment plant, after having subjected said concentrates to a second coagulation/flocculation step. 
     In practice, the sludges tested had a suspended solids concentration of between 126 mg/l and 394 mg/l. 
     The phosphonate elimination rate of these concentrates was assessed according to the total phosphorus elimination (P total ) in the concentrates treated. 
     The results obtained were compared to those obtained by an identical process, but not including the step consisting according to the invention of placing the concentrates having undergone a second coagulation/flocculation steps in contact with the primary sedimentation sludges coming from the potable water treatment station. 
     For better reliability of the analyses, the elimination of phosphonates was determined according to the total phosphorous elimination (P total ). 
     The results of these tests are presented in the graph of  FIG. 2 . 
     It is observed that, for the same percentage of total phosphorous elimination P total , the coagulant (FeCl 3 ) doses used are more reliable when an adsorption step is performed according to the invention. 
     Thus, to remove 75% of the total phosphorous, it is necessary to use 60 ppm of FeCl 3  with sludge at 126 mg/l (suspended solids) by comparison with 150 ppm of FeCl 3  without the adsorption step.