Chapter 14: Treatment processes, filtration and adsorption

Chapter 14: Treatment processes, filtration and adsorption

Contents

14.1 Introduction 1

14.2 Diatomaceous earth filtration 2

14.2.1 Vacuum or standard DE filtration 3

14.2.2 Pressure or modified DE filtration 4

14.2.3 Some operating issues with DE filtration 5

14.2.4 Monitoring 5

14.3 Slow sand filtration 6

14.3.1 Cleaning 7

14.3.2 Monitoring 8

14.3.3 Aeration 8

14.3.4 Some operating issues with slow sand filtration 9

14.4 Membrane filtration 9

14.4.1 Introduction 9

14.4.2 Current experience in New Zealand and overseas 10

14.4.3 Fundamentals of membrane filtration 10

14.4.4 Membrane selection 13

14.4.5 Membrane plant operations 14

14.5 Cartridge filtration 20

14.6 Bag filtration 26

14.7 Adsorption processes 28

14.7.1 Activated alumina 28

14.7.2 Solid block activated carbon (SBAC) filters 29

14.7.3 Granular activated carbon (GAC) filters 29

14.7.4 Biologically active filters (BAC) 30

14.8 Desalination 30

References 31

List of tables

Table 14.1: Properties of typical membrane materials 13

Table 14.2: Typical design/operating criteria for MF/UF systems (guidance only) 19

Table 14.3: Data log and check sheet 19

Table 14.4: Chemical cleaning log sheet – provided by membrane supplier 20

List of figures

Figure 14.1: Diatomaceous earth pressure plant at Mokau, Waitomo District 5

Figure 14.2: Typical submerged membrane system 12

Figure 14.3: Cutaway showing cartridge seal 23

Figure 14.4: Suggested arrangement for reading pressure differential across a cartridge filter 24

Figure 14.5: Typical pressure drop across a cartridge during a filter run 24

14.1 Introduction

Chapter 13 discusses issues relating to the operation of water treatment plants using a chemical coagulant such as alum or polyaluminium chloride followed by rapid granular media filtration. This process may also include sedimentation (ie, clarification) or dissolved air flotation. Filters following coagulation processes operate mainly by adsorption processes rather than straining or entrapment. Chapter 13 also briefly describes water softening using lime (followed by sand filtration) because it is a process that the USEPA and the DWSNZ considers capable of removing protozoa.
This chapter discusses diatomaceous earth filtration, slow sand filtration, membrane filtration, cartridge filtration, and bag filtration. These are the other water treatment processes that can remove Cryptosporidium oocysts effectively enough to be considered for protozoa log credits. Their filtration process operates by straining or entrapment.
This chapter also discusses adsorption processes that do not need to follow coagulation processes. These can remove some of the chemical determinands with MAVs. Adsorption processes are also discussed in Chapter 18 with reference to taste and odour control, and in Chapter 19, mainly related to point-of-use and point-of-entry treatment systems. Activated carbon is also mentioned throughout Chapter 9, as a means of adsorbing cyanotoxins from water.
No other filtration processes are discussed in this chapter. Chapter 12, on pretreatment processes, includes some commentary on screens and other coarse filtering processes. These do not qualify for any protozoa log credits.
The 2008 DWSNZ include a new section, section 5.17: Alternative processes: treatment compliance criteria, whereby water suppliers may apply to the Ministry of Health to have other treatment processes assessed for a log credit rating. This approach, which is explained more fully in section 8.4.5 of the Guidelines, allows water suppliers to apply for a log credit rating (or a variation to the prescribed log credits) for a treatment plant or process:

a) not covered in sections 5.1–5.16 of the DWSNZ

b) that performs demonstrably better than its compliance criteria

c) that performs to a lesser, but reliable, level than specified in its compliance criteria.

This chapter concentrates on the operations and management of the processes; Chapter 8 discusses their compliance issues with respect to protozoa removal.
The bag and cartridge filtration sections have been expanded because they are being used more often since when the 1995 Guidelines were produced, and because more experience has been accumulated in recent years regarding their use.
The membrane filtration section in the 1995 edition of the Guidelines was just one paragraph. Technological advances have resulted in the process being used much more often today. Consequently, this section is now quite large.
The other treatments, slow sand and diatomaceous earth filtration, have been expanded because they may be attractive processes for smaller supplies for protozoa removal.
Some process variation is normal and expected; however, too much variability can result in filtration failures, leading to waterborne disease outbreaks. An objective of the DWSNZ, therefore, is to keep process variability within acceptable limits. Understanding the causes of process variations should prevent recurrences.
The AWWA has produced manuals on precoat filtration and on reverse osmosis/nanofiltration, see references. The full list of AWWA manuals and standards appears on www.awwa.org/files/Resources/Standards/StandardsSpreadsheet.xls.
Risk management issues related to the filtration processes in this chapter are discussed in the:

MoH Public Health Risk Management Plan Guide PHRMP, Ref: P6.2. Filtration – Slow Sand Filtration.

MoH Public Health Risk Management Plan Guide PHRMP, Ref: P6.3. Treatment Processes – Cartridge Filtration.

MoH Public Health Risk Management Plan Guide PHRMP, Ref: 6.4. Filtration – Diatomaceous Earth.

MoH Public Health Risk Management Plan Guide PHRMP, Ref: P6.5. Treatment Processes – Membrane.

MoH Public Health Risk Management Plan Guide PHRMP, Ref: P10. Treatment Processes – Pump Operation.

MoH Public Health Risk Management Plan Guide PHRMP, Ref: P11. Treatment Processes – Plant Construction and Operation.
DWI (2011) has prepared a list of products that are approved for use in UK water supplies.
A section on desalination has been added because this chapter appears to be the most appropriate. Desalination is not used yet in New Zealand, and has not been considered directly for protozoal compliance.