Membrane technology

Filtration by itself and in combination with other physical or chemical processes has been used in water treatment for years. In fact, water treatment started with filtration only, and then other processes were introduced upstream and downstream of the filters in order to enhance the final water quality.

While the word “filtration” means simply “a technique for separating impurities from a liquid,” there is nothing simple about filtering water through a single media (sand or anthracite, or activated carbon), or through a multi-media granular bed filter (various combinations of sand, anthracite, activated carbon, garnet, etc.).

Only a minor part of a filter’s performance is due to its physical rejection of particles from a fluid. Most of these large void-type filters perform largely by complicated and mostly empirical processes of chemical and electrical bonding, attraction and absorption of impurities by the granular material. Since the size of pores (actually the size of voids between granular particles of the media) is relatively large, granular media filters usually require a very small input of energy. In some cases granular media filters operate in a gravity mode with as little as 1.0 metres of water depth above.

Although the process of filtration through granular media was never completely understood, a number of theories and empirical formulas were developed over the years that provide a basis for the design of water treatment plants.

Lately, however, a tendency has developed to use membrane filters instead of granular media filters. There are several reasons for membrane filters’ recent popularity:

— the development of reliable and long-life membranes

— a reduction in the capital and operating cost of the units

— the availability of membrane filtration packages from manufacturers.

In the membrane filtration process, a liquid containing impurities is passed over a physical barrier — a porous membrane. The liquid and the impurities that are smaller than the pores pass the membrane, while the larger particles are rejected.

Membrane filtration covers a wide range of filters with various pore sizes:

microfiltration (MF) — 0.1 to 1.0 micrometre pores. Microfiltration is effective in removing suspended solids which are not easily settled, and micro-biological content like bacteria, cysts and oocysts.

ultrafiltration (UF) – 0.01 to 0.1 micrometre pores or 20,000 to 100,000 molecular weight cutoff (MWC). Ultrafiltration operates in a smaller filtration range but does not remove dissolved organic or inorganic contaminants.

nanofiltration (NF) — 0.001 to 0.01 micrometre pores or 1,000 to 20,000 MWC. Nanofiltration removes most dissolved organic matter but not dissolved inorganic solids.

reverse osmosis (RO) — 0.0001 to 0.001 micrometre pores or up to 1,000 MWC. Reverse osmosis has traditionally been used for removing dissolved salts, primarily as a desalination process.

Due to the small size of the pores, membrane filtration is a high energy process that requires pumps in order to generate enough pressure for the liquid to permeate the membrane. Obviously, the smaller the size of the openings, the higher the energy input required. In fact, microfiltration membranes operate with a pressure differential of approximately 275 kPa (40 psi) while reverse osmosis requires a water pressure of as much as 2,070 kPa (300 psi) and higher.

Furthermore, the smaller the spore size, the more filtration area is needed, thus requiring larger and more sophisticated equipment and higher capital and operating costs.

Until recently the costs were prohibitive and the major reason municipalities did not use membrane filtration in water treatment. The situation has changed, however, with the latest advances in manufacturing technologies.

Pilot studies in Sudbury and Sioux Lookout

Two years ago Proctor and Redfern was involved in two very interesting projects — testing for the development of new water supplies for Sudbury and Sioux Lookout.

Both municipalities are in Northern Ontario. The quality of the available source water was typical to the area: water in small northern Ontario lakes generally has low turbidity (between 0.5 and 5.0 nephelometric turbidity units or NTU), very low natural alkalinity, and is hard. It has a relatively high organic content and an elevated colour that ranges between 25 and 250 platinum-cobalt units or PCU. For comparison, the Ontario Ministry of the Environment stipulates that the turbidity of drinking water must not exceed 1.0 NTU, while colour is maintained below 5.0 PCU.

Proctor and Redfern and other consultants’ previous experience with such source water pointed towards conventional water treatment trains — chemical coagulation and flocculation followed by sedimentation and filtration through granular multi-media filters. We have completed bench scale testing of this type of treatment for both locations to ensure that the selected processes are capable of purifying the lake water to drinking water quality standards. In both cases, the addition of aluminum sulphate or alum — a chemical coagulant — and filtration aid resulted in a filtrate quality that met the requirements. The feed rate of alum required ranged from 25 milligrams per litre (mg/L) in Sudbury to as much as 60 mg/L in Sioux Lookout.

During the treatability study in Sioux Lookout, however, nature threw a monkey wrench into the process. In the spring of 1997 the presence of Giardia cysts and Cryptosporidium oocysts were identified first in the town’s existing distribution system (an unfiltered supply) and then in Pelican Lake, the source of the town’s water supply. A well designed conventional water treatment plant can remove up to 99.99% of the parasites. It cannot, however, guarantee complete removal. The town council, concerned with the possible health effects, decided to try and completely remove the parasites within the water treatment train.

That is when microfiltration came into play. We knew that microfiltration is a permanent barrier to bacteria, cysts and oocysts, as well as any suspended matter. We also knew that microfiltration by itself would not remove some of the dissolved contaminants and would require chemical pre-treatment — thus transforming the process to chemically-assisted microfiltration.

What we did not know was how microfiltration would react to the injection of coagulants that were required for removing the colour-causing dissolved organic compounds in the source water. For this part of the study Proctor and Redfern rented two microfiltration test units — a pressure driven bench scale unit from one manufacturer and a vacuum-driven pilot unit from another.

Besides their production capacity, the main difference between the two units is how they create energy for passing the liquid through the membrane. In both cases, the pressure differential between the dirty (higher pressure) and the clean (lower pressure) sides is the main driving force of the process.

In order to gain a better feel for the process, we evaluated several different coagulant formulations — alum, polyaluminum chloride, polyaluminum silicate sulphate, as well as a combination of alum with sodium aluminate — at various feed rates. With any chemical, the main challenge of the coagulation state was to increase the production of “pin” floc (small size, light weight, not settleable) and decrease the creation of large size easily settleable floc which is required for conventional water treatment. We achieved this by reducing the time of flocculation and increasing the energy input

By analyzing the filtered water test results we concluded that coagulation with alum at a feed rate of approximately 40 milligrams per litre followed by microfiltration performed the best. Both microfiltration units produced water of an acceptable quality, allowing us to recommend the use of chemically-assisted micro-filtration to the town.

The terms of the study did not enable us to recommend any particular commercial equipment. Also, due to the limited duration of the study we could not evaluat
e the long-term effects of chemical pretreatment on membrane fouling nor the performance of the units in extremely low temperatures (+0.2C), which would be typical in the winter operation.

We also conducted a study of this emerging technology in Sudbury using only one pressure-driven pilot scale microfiltration unit. Although the microfiltration process was not completely optimized due to time limitations, the test run indicated we might reduce the alum feed rate to 20-25 mg/L for microfiltration units, versus 40 mg/L for conventional water treatment.

Based on these results, it is obvious that chemically assisted microfiltration is a feasible water filtration alternative to conventional water treatment in general, and to granular multi-media filters in particular. As well as effectively removing suspended solids and colour-causing dissolved organic matter, microfiltration can provide a permanent barrier to Giardia cysts and Cryptosporidium oocysts in the raw water.