IMMERSED MEMBRANE FILTRATION CYCLE WITH EXTENDED OR TMP CONTROLLED CYCLE TIME

Abstract

A filtration process has permeation periods and deconcentration periods. The length of the permeation period or periods between backwashes is chosen, or controlled, to allow the system to reach a TMP and flux value at which the system is operating near its mechanical limits.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

One or more processes or apparatuses will be described below with reference to the following Figure(s).

FIGS. 1 and 2 are graphs of experimental results.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover processes or apparatuses that are not described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or process described below is not an embodiment of any claimed invention. The applicants, inventors and owners reserve all rights in any invention disclosed in an apparatus or process described below that is not claimed in this document and do not abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.

Experiments were run with immersed hollow fiber membranes, operated according to a batch process with a generally constant flux and within the parameters of paragraph 9. Long cycle times, for example up to 10 times or more longer than in traditional batch processes, were used and the TMP was allowed to reach a set TMP of 70-90% of maximum mechanically allowable TMP before backwashing was conducted. It was found when operated in parallel with an identical membrane and feed water using the traditional batch mode of operation that there is a performance advantage to running with long cycle times and a TMP triggered backwash instead of a time triggered backwash. The membrane TMP (measured after backwashing) increased more slowly in the TMP triggered backwash method (FIG. 1). In addition, due to the increased ratio of time permeating to time backwashing, there was an increase in net flow of water, or recovery rate, separately from the performance advantage. The combination of the increase in net flow and decrease in fouling rate makes this operating method very attractive. An additional advantage was a reduction in waste volume resulting from less frequent drain steps.

A potential concern with long filtration periods is that operating at high recoveries increases the solids levels towards the end of a dead end filtration cycle. Depending on the particular membrane module, solids tolerance boundaries may be breached. Accordingly, a solids tolerant module may be desirable. Further, an effective method of cleaning is also desirable. In particular, a cleaning and deconcentration procedure may involve aerating while slowly draining the tank or draining the tank in steps, for example to water levels corresponding to areas prone to sludging or to step through the entire depth of a module, and aerating for a period of time at each step. Some such desludging procedures are described in U.S. Publication No. 2006-0065596 A1 which is incorporated herein, in its entirety, by this reference to it. Alternately, maximum concentration may be reduced by a full or partial drain or flush (that is opening a drain but also increasing feed below to keep the membranes immersed) between backwashes or by adding a retentate bleed during the entire, or a later part, of the filtration period.

The results of the experiments discussed above are provided in the following Tables 1 and 2 and FIGS. 1 and 2.

TABLE 1
Process Operating Details of Side-by-Side Membranes Operated at
Same Instantaneous Flux (30 gfd)
Traditional Batch Process for
Immersed Membranes	TMP Triggered Backwash Process
		Duration			Duration
Step	Process	(minutes)	Step	Process	(minutes)
1	Start Permeation	30	1	Start Permeation	120–300
					(continues
					until
					TMP = TMPmax
2	Stop Permeation,	0.5	2	Stop Permeation,	0.5
	Start Aeration, Start			Start Aeration,
	Backpulsing			Start Backpulsing
3	Stop Backpulse,	1.5	3	Stop Backpulse,	1.5
	Continue Aeration,			Continue
	Drain Tank			Aeration, Drain
				Tank
4	Stop Aeration, Fill	1.5	4	Stop Aeration, Fill	1.5
	Tank			Tank

TABLE 2
Process Performance Analysis
Traditional Batch Process for
Immersed Membranes               TMP Triggered Backwash Process
Net Flux = 27 gfd                Net Flux = 29.7 gfd
% of Feed Water Recovered = 87.5% % of Feed Water
                                 Recovered = 97.2%
% of Feed Water Wasted = 12.5%   % of Feed Water Wasted = 2.8%
Fouling Rate = 0.4 kPa/d         Fouling Rate = 0.1 kPa/day (over 3
(over 3 weeks)                   weeks)

The invention or inventions which are currently claimed in this document are described in the following claims.

Claims

1. An immersed membrane filtration process in which the length of one or more filtration periods between subsequent backwashes is chosen or controlled such that, immediately prior to a backwash, the system is operating at a combination of TMP and flux values near the mechanical limits of the system.

2. The process of claim 1 wherein flux is generally constant.

3. The process of claim 1 wherein TMP immediately prior to a backwash is 8 psi or more.

4. The process of claim 1 wherein the time between at least some pairs of subsequent backwashes is 60 minutes or more or 90 minutes or more.

5. The process of claim wherein 1 time between at least some pairs of subsequent backwashes is determined by reaching a predetermined value in the range of 70 to 100% of a design maximum TMP while operating at a generally constant design flux.

6. The process of claim 1 further comprising a deconcentration step after or during the backwashings.

7. The process of claim 1 comprising air scouring the membranes while draining a tank, or while the surface of the water in the tank is at a plurality of heights passing through the membranes.