Chapter 14: Treatment processes, filtration and adsorption

14.2 Diatomaceous earth filtration

Diatomaceous earth filtration uses a mobile material to build up a filter wall on a membrane. Diatomaceous earth (DE) is a fine, powdery substance comprising the skeletons of diatoms (microscopic algae). It occurs as natural deposits, which are mined, dried, graded and bagged. The usual source is the USA.
DE has been used in New Zealand commonly for swimming pool filtration and in the food industry; for example, most breweries use it to ensure that no yeast is carried over. DE filtration does not remove much colloidal colour or soluble organic matter. These materials are too small to be captured by the mechanical filtration process of DE. They require much finer filtration, or coagulation to allow them to be agglomerated into a floc. DE filtration is mainly used to treat clean stream waters and springs and is accepted in the DWSNZ (Ministry of Health 2005, revised 2008) as being capable of earning 2.5 log credits for protozoa removal.
As at 2005, four water supplies (Ohakune, Woodville, Mokau and Benneydale) have been using a DE process for municipal supply. Bonny and Cameron (1998) described the Woodville plant.
The DE material varies in size. Larger diameter material causes less headloss through the filter layer but offers less protection against protozoal (oo)cysts or other particulate matter. ANSI/AWWA B101-12 covers precoat filter media.
Typically, the finer DE material is around 15–20 microns median size and the coarser material is around 35–40 microns. Both contain a wide range of sizes, but the uniformity coefficient is normally about 5. The uniformity coefficient, or UC, is the ratio between the material’s d₆₀ and its d₁₀ with the d₆₀ being the particle size that 60 percent of the material is smaller than, and d₁₀ having a corresponding meaning. The pore sizes (the holes between the DE particles) range from about 5 to about 12 microns. The DWSNZ do not specify a maximum median size; compliance is based on performance, as measured by turbidity. Ogilvie (1998) described diatomaceous earth and its use in filtration.
Ongerth and Hutton (1997) found that at least 3 log removal of Cryptosporidium was achieved using the coarser media at low flow rates (2.4 m/h). Finer media and higher flow rates (4.9 m/h) improved the results to around 6 log. The improved filtration at higher filtration rates is due to compression of the filter cake. Local practice is to operate at about 4.3 m/h using DE that is rated to remove particles down to 1.2 microns.
WHO (2004a) calls this process ‘precoat filtration’, and reports some interesting developments:

Precoat filters remove smaller microbial particles (eg, bacteria and viruses) less effectively than they do parasites, unless the coating materials are chemically pretreated; for example, with aluminium or iron coagulants, or with cationic polymers. In a pilot study by Schuler and Ghosh (1990), removal of coliforms with untreated DE was about 0.36 logs, increasing to 0.82 logs with a coating of alum at 1 mg/g DE, and to 2 logs at 3 mg/g DE. This increase was probably due to the trapping of bacteria by the alum. A similar beneficial effect was observed using cationic polymers; at 3.5 mg/g DE, removal of coliforms increased to 3.3 logs. The authors concluded that this increase in removal could be due to an increased site density on the polymer-coated DE for adsorption of negatively charged coliform cells. A similar improvement in removal of bacteria was reported for the pilot study conducted by Lang et al (1986). Alum coating of DE increased removal of total coliforms from 0.16 logs to 1.40 logs, and of HPC bacteria from 0.36 logs to 2.30 logs. Removal of viruses also increased with chemical pretreatment of filter cake (Brown, Malina and Moore 1974). The removal of bacteriophage T2 and poliovirus was about 90 percent (1 log) for an uncoated filter, but increased to more than 98 percent (1.7 logs) when the filter cake was coated with ferric hydrate or polyelectrolytes.

14.2.1 Vacuum or standard DE filtration

The DE is introduced into the water stream by a dosing pump drawing from a stirred DE slurry tank. The concept is to capture the DE particles on a membrane and use them to build up a filter wall. The membrane, a heavy linen type of material, surrounds a solid base called a septum. This usually consists of ABS or PVC tubes with 2–3 mm holes in the walls that allow the filtered water to enter, where it is collected and passed to the next stage.
To create a DE coat on the membrane, a high initial dose is applied, which quickly (say in 20 minutes) builds up a layer of perhaps 2–3 mm thick. This stage is known as the precoat stage and the water during this stage needs to be recirculated until full filtration is established. The amount of precoat applied depends on the filtering surface area. The precoat is measured in kg/m²; the normal precoat dose is about 1 kg/m².
During the filter run (ie, the time the filter operates before the DE must be washed off and the septum recoated), a small maintenance dose of DE is added to the incoming water. This is called the body feed. Its dose rate is based on the turbidity of the raw water and filtration rate, and must be determined by experience. As a guide, fairly clean raw water can be dosed at about 0.15 kg/m²/day. This needs to be increased as the turbidity increases. The filtered water should (in theory) contain no particles larger than about 2–3 microns (micrometres). DE therefore provides an effective barrier against the (oo)cysts of Giardia and Cryptosporidium.
The optimum filtration rate is about 0.8 L/s/m² (2.9 m/h) with a maximum of 1.2 L/s/m² (4.3 m/h).

14.2.2 Pressure or modified DE filtration

Many newer DE plants are contained inside a pressure vessel. The concept is similar to that of vacuum DE systems but varies in that:

the precoat can be applied as quickly as five minutes; again to a thickness of 2–3 mm

there may be no body feed, although if there is any suspicion of cracking or shrinkage of the cake, body feed should be used

the filter run time is usually shorter than vacuum filtration due to the higher filtration rate. The optimum filtration rate is about 1.25 L/s/m² (4.5 m/h) with a maximum of 1.6 L/s/m² (5.8 m/h)

there may be provision for a drop coat procedure, where the DE coat is backwashed off and then re-applied, without removing it from the vessel. This technique increases the risk of recycling previously trapped protozoa, thereby lowering drinking-water quality. Another reason for not using the drop coat technique is because fine clays etc become embedded in the filter support or element cloth, shortening filter runs, and requiring more frequent overhauls. Overhauls involve taking the top off the filter, removing the elements, waterblasting them, and reassembling the unit.
Figure 14.1: Diatomaceous earth pressure plant at Mokau, Waitomo District

Courtesy of Filtration & Commercial Pumping Ltd.

14.2.3 Some operating issues with DE filtration

Establishing the pre-coat: During this time filtered water must be recycled.

Body feed: The idea behind a continuous body feed is to supply loose DE to plaster over any cracks that develop in the pre-coat. These cracks are possible, given the flexible substrate of the membrane. The continuous feed also ensures that the porosity is maintained.

Filter run time: Limitations on the filter run time are usually caused by accumulated headloss. With clean feed water (say, under 2 NTU), headloss will build up at between 0.06 and 6 m/day. For example, where the maximum headloss allowed is 4 metres, the filter run time may be between one day and several weeks.

DE handling and disposal: DE is a siliceous material and can cause respiratory problems if inhaled in dry form. Care must be taken with handling procedures, including removal and disposal (normally to landfill) of spent material.

14.2.4 Monitoring

The DWSNZ use turbidity as an operational requirement in place of monitoring for protozoa against the MAV and the monitoring requirements are described in the DWSNZ.

Should the turbidity exceed these requirements the operator should check whether:

the DE dose is appropriate for the raw water conditions

the cake has built up enough before drinking-water is produced

the treatment rate through each filter is within specification

the filter cake has shrunk or cracked, ie, whether the body feed is appropriate

there is any short-circuiting

the raw water quality has changed.

It is recommended that water suppliers establish a control limit for each MAV or operational requirement. Control limits are discussed in Chapter 17: Monitoring. The preventive actions that are to be considered when a control limit is reached are to be documented in the WSP. The purpose of control limits and the preventive actions is to avoid reaching any transgression levels or operational requirements. For example, a control limit for turbidity of the water leaving each filter set at about 0.25 NTU may be advisable.