DESIGN OF THERMOPHILIC DIGESTERS AT THE CHRISTCHURCH WASTEWATER TREATMENT PLANT

Humphrey Archer, Reuben Bouman, Graeme Wells, CH2M Beca Ltd Mike Bourke, Christchurch City Council

ABSTRACT

The Christchurch Wastewater Treatment Plant (CWTP), which serves a population equivalent of 450,000, has recently constructed two 7,000 m³ digesters capable of operating at mesophilic or thermophilic temperature, adding to the existing four 5,000 m³ digesters that operate at mesophilic temperature. This paper discusses a number of design issues and features of the two new digesters including:

• Anticipated benefits of thermophilic digestion, such as increased solids destruction, increased biogas yield, and disinfection of sludge

• Gas injection mixing instead of mechanical mixing, and use of CFD model to optimise mixing energy

• Serpentine tube heat exchangers for heat recovery from thermophilic sludge and transfer to raw sludge rather than spiral types, to reduce clogging potential

• Sludge rheology and pump selection for high head and chopping duties

• Instrumentation required for fully automatic control

KEYWORDS

Thermophilic, Temperature Phased, Digestion, CFD, Heat Exchangers, Sludge Pumps, Sludge Viscosity

1 INTRODUCTION

Since the Christchurch Wastewater Treatment Plant (CWTP) was commissioned in 1961, it has used mesophilic anaerobic digestion for sludge stabilisation. Biogas has been used in engines coupled to pumps or to alternators for electricity generation, and in boilers for sludge heating.

Additional sludge quantities, from 2002 to 2005 due to increased industrial loads and an enhanced secondary treatment process, have resulted in marginal retention times in the four 5,000 m³ mesophilic digesters. This resulted in a digestion process upset in September 2002, with release of odours, and ongoing intermittent foaming. To cater for the increased loads and future growth, more digestion capacity was required.

A Net Present Value comparison was made to see what advantage there would be in constructing two new 7,000 m³ digesters now, rather than building two 5,000 m³ digester now with plans to build another in the future. This analysis concluded that the increased gas production and reduced sludge disposal costs associated with the 7,000 m³ digesters when compared to 5,000 m³ digesters, justified the higher capital cost. Thus it was decided to construct two new 7,000 m³ digesters (Digesters 5 and 6) which could be operated at mesophilic or thermophilic temperatures. With six digesters in total, the plant would have sufficient redundancy to take a digester out of service without risking process failure.

2 THERMOPHILIC DIGESTION

2.1 PROCESS DESIGN In Europe and USA, use of thermophilic digestion, either on its own or upstream of mesophilic digesters, has been increasing over the past 20 years. Claimed benefits of thermophilic digestion are increased solids destruction with increased biogas and less solids for final disposal, plus reduced foaming, all achieved at reduced retention times. Another significant benefit of thermophilic digestion is the reduction of pathogens at the 55°C operating temperature. The first operating thermophilic digester in Australasia, was at the Green Island WWTP (Dunedin) in 2001, and Christchurch is the second installation.

The process design for the CWTP was Temperature Phased Anaerobic Digestion (TPAD). The new digesters would run in parallel at thermophilic temperature followed by the four existing digesters in parallel at mesophilic temperature (This is known as Mode B). The claimed benefits of the TPAD process are the combination of the greater digestion rate offered by the thermophilic process coupled with the stability and less odorous sludge produced by the mesophilic process. Table 1 shows the solids retention time and volatile solids loading rates.

Table 1: Digester Capacity and Hydraulic Retention Times

Initial 2008 Flow Rate Design 2026 Flow Rate Parameter (a)

Average Month

Max. Month

Average Month

Max. Month

Design Target /

Range (d)

Sludge flow to digesters (4.5% average solids content), m³/d 940 1100 1250 1400 N/A

Retention with 6 digesters in service, days (T/M) (b)

31.9 (13.8/18.1)

27.3 (11.8/15.5)

24.0 (10.4/13.6)

21.4 (9.3/12.1)

20

Retention with 5 digesters in service (digester 5 or 6 out of service), days (T/M) (b)

25.0 (6.9/18.1)

21.4 (5.9/15.5)

18.8 (5.2/13.6)

16.8 (4.6/12.1)

20

Volatile solids loading rate with 6 digesters in service in parallel, kg/(m³.d) (c) 1.13 1.38 1.50 1.68

1.3 ≤ ≤ 1.9 to 2.5

Volatile solids loading rate with Digester 5 or 6 out of service, (kg/m³.d) (c) 1.44 1.69 1.91 2.10

1.3 ≤ ≤ 1.9 to 2.5

Volatile solids loading rate on lead digesters, Digesters 5 and 6 (T) 2.60 3.05 3.46 4.02

Notes: (a) Effective volume less than nominal volume. (b) T/M stands for: Thermophilic/Mesophilic (c) Assumes all mesophilic digesters operating in parallel and VS being 80 % of TS. (d) North American design targets for digester retention time assume that part of the digester is unavailable due to deposition of solids.

The design allows for several other operating modes including conventional single stage mesophilic digestion with all six digesters in parallel (known as Mode A), and two stage thermophilic digestion (i.e. Digesters 5 and 6 in series) followed by mesophilic digestion (known as Mode D). Initially Mode D was the preferred operating mode, as this could provide greater pathogen reduction, likely to give Grade A quality biosolids, because the thermophilic digesters are in series and there is reduced risk of short-circuiting. However research showed that a short retention time in the first digester is likely to lower the methane composition in the biogas, see Figure 1, (as this digester moved closer to an acid phase digestion) resulting in unstable gas engine performance.

2.2 EXAMPLE PLANTS 2.2.1 INTERNATIONAL

At the time of design Temperature Phase Anaerobic Digestion had been used in Germany for 20 years at approximately 15 plants, and more recently in the USA at about 5 plants.

38

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

0 2 4 6 8 10 12 14 16 18 20Retention Time (days)

Met

hane

in B

ioga

s (v

ol%

)

Dichtl - MesophilicDichtl - ThermophilicGhosh - MesophilicGhosh - ThermophilicCWTP - MesophilicHan et al - MesophilicHan et al - ThermophilicTerminal Island - ThermophilcChicago WSW - MesophilicChicago WSW - ThermophilicAnnacis Island - ThermophilicGreen Island - ThemophilicThermophilic Best FitMesophilic Best Fit

Mode D Mode B

Lower Limit for Engine Stability

Figure 1: Biogas Composition from various Pilot and Lab Scale, and Full Scale Plants

Oles et al (1997,) state that shorter thermophilic retention yields a better blend of volatile acids than longer retentions. Thermophilic digesters with a retention time of 2 to 3 days produce some methane, as shown by figures 4 and 5 in Dichtl (1997). However, most of the methane is produced in the mesophilic digesters from the volatile acids formed in the thermophilic digester. The thermophilic biogas is blended with the mesophilic biogas in the same gas piping system. The Green Island experience supports the German data.

USA thermophilic digesters tend to have 5 to 10 days retention or longer. The Omaha WWTP converted an existing digester to thermophilic operation and had stable performance with 5 days retention in the thermophilic digester and 15 days in the mesophilic digesters. Process information for plants that have changed from purely mesophilic digestion to TPAD is presented in Table 2. The median increase in volatile solids reduction is 16 %.

2.2.2 NEW ZEALAND

Temperature Phase Anaerobic Digestion has operated reliably at the Green Island WWTP since early 2001, and this is the only operating municipal thermophilic digester in Australasia. The Green Island WWTP catchment has a domestic population of 14,000 and industrial loads up to 80,000 population equivalent. Due to seasonal variability and holidays, the industrials loads can increase rapidly. The thermophilic digester has handled the rapid load changes without loss of stability.

The Green Island WWTP does not have to produce a defined standard of biosolids, and pathogen indicator reductions are not routinely measured. From six sampling runs done in 2002, these were the results for faecal coliforms as medians (MPN/100 ml):

• Raw sludge 50,000,000

• After thermophilic 300

• After mesophilic < 20

Table 2: Process Data for Plants that have converted from Mesophilic to TPAD

Plant % Volatile Solids Reduction (VSR) Retention Time (days) Mesophilic TPAD % Change Thermo. Meso. Total WLSSD, MN (b) 40 46 15 8 23 31 King Co, WA (b) 58 68 17.2 8 16 24 Neenah-Menasha, WI (b) 50 58 16 16 16 32 Madison, WI (b) 58 64 10.3 5 15 20 Hyperion WWTP, CA (b) 48 66 37.5 13 20 33 Cologne, Germany (b) 34 43 26.5 7 27 34 Osterode (c) not stated 55 not stated 4 14 18 Geseke (c) not stated 47 not stated 4 55 59 Auenheim (c) not stated 60 not stated 3 15 18 Erkelenz (c) not stated 60 not stated 2.6 19 21.6 Altenmarkt (c) not stated 54 not stated 3.5 17 20.5 Koln-Stammheim (c) not stated 60 not stated 5.5 21 26.5 Median Values 49 59 16 5.3 18.0 25.3 Proposed CWTP (d) 10.4 13.6 24

Notes: (a) Taken from Table 1. Advanced Anaerobic Digestion Process Information from the WEFTEC 2002 paper ‘Advanced Anaerobic Digestion Performance Comparisons1, by Schafer, Farrel, Newman and Vandenburgh

(b) Taken from Table 2. Results of full scale ASTM plants from the 1997 paper ‘Full Scale Experience of Two Stage Thermophilic/Mesophilic Sludge Digestion’, by Oles, Dichtl and Niehoff

(c) Values for Christchurch Wastewater Treatment Plant are based on 2026 average sludge flows, Digesters 5 and 6 constructed with nominal 7,000m³ capacity and all digesters in service using Operation Mode B

3 DIGESTER MIXING

3.1 MIXING OPTIONS A wide variety of mixing technologies have been used elsewhere, with the key choices being:

• Gas injection OR propeller OR pumped hydraulic nozzle.

• Unconfined OR draft tube.

• Vertical toroidal roll OR mixed horizontal and vertical currents.

The positives and negatives of the mixing options listed were investigated and it was concluded that alternative mixing systems did not offer benefits that would outweigh the long history of trouble free operation experienced with the mixing of the existing digesters. The worldwide average interval between digester clean-outs (to remove settled sediments and scum mats) is 8 years. The existing digesters at CWTP have operated for 15 to 18 years between clean-outs, which results in significant savings.

Having reviewed the experience of a wide variety of mixing systems at other plants, it was recommended that the mixing system used in existing digesters, be retained for the new digesters with a number of enhancements such as the use of a spill weir at the surface of the digester to allow the removal of scum mat material and foaming organisms from the digester.

Hence, the new digesters are mixed by a combination of the following three methods, which provide standby (or assist mixing), if one system is out of service.

i. Confined central gaslifter within an eductor tube (as in existing digesters) complete with rotation vanes and deflector cone.

ii. Unconfined gas injection from 12 unconfined lances in two peripheral rings at about 0.5 and 0.75 radius (as six pairs of nozzles, normally sequentially operated in groups of four nozzles at a time, with the central gaslifter switched off).

iii. External pumped system with low power hydraulic injection into the bottom of the eductor tube, which can be operated in conjunction with (a) and (b) or on its own.

These mixing systems have the following features:

i. Confined Mixing • There are two forces creating vertical stratification in a digester – temperature differential and biogas

buoyed flotation of solids. A central gaslifter eductor tube can vertically mix thus overcoming stratification by creating a symmetrical toroidal roll current.

• The eductor tube has helical vanes to create horizontal rotation to further assist mixing and minimise the potential for dead spots. This rotational movement at the surface aids the removal of surface scum at the discharge weir.

• A deflector cone has been included to create a stronger roll current as in the existing digesters. Without a deflector cone at the top of the central gaslifter tube, the upwards flowing "boil" is not efficiently converted to outwards radial currents.

ii. Unconfined Mixing • The central gaslifter can mix the central zone efficiently but may not fully mix the perimeter and mid zones

as effectively, which the unconfined gas lances will do.

• Unconfined gas lances have the advantage of causing uplift and mixing of the digester contents, before “boiling” the surface of the digester to assist in breaking up scum mats. These lances are normally operated intermittently in a sequence around the digester, rather than all continuously (i.e. similar in operation to the Pearth tube system) used in the four existing digesters. The sequenced operation concentrates more agitation in one location and the asymmetrical pattern produces both vertical and lateral movement. The lances will be cover mounted. These will be fed from the rotary lobe type gas blowers discharging at the same depth as those within the eductor tube.

iii. Pumped Re-circulation • There are two re-circulation systems. One is specifically designed for mixing; this is the main re-circulation

system. The other is known as auxiliary re-circulation, and used for sludge heating.

• Main pumped re-circulation has been given increased capacity (relative to existing digesters), so that improved alternative mixing can be operated continuously, if the gas mixing needs to be intermittent to control foaming

• Sludge extraction and return points for the re-circulation system is at two levels, at equi-spaced at six locations around the perimeter of the digester with provision for a further six. When operated as return points, the upper level assists in the breaking up of scum and the lower level locally disturbs any settled deposits.

• As in the existing digesters, raw sludge will be fed into the pump re-circulation pipeline which discharges upwards inside the base of the central eductor tube to achieve good initial mixing with the digester contents – bringing the biomass in close contact with the feed sludge.

3.2 COMPUTATIONAL FLUID DYNAMICS A CFD (computational fluid dynamics) study was carried out to determine the effectiveness of the mixing system, and investigate possible reductions in air injection rates, thus saving power. The major findings of this study were:

• The flow pattern is a single toroidal roll current with its core at about two thirds radius (see Figure 2)

• Gas injection rate could be decreased from 650 l/s to 420 l/s saving approximately 730 kWh/d

• Helical vanes on the educator tube (see Photograph 1) should be retained but with a start angle of 45° as opposed to vertical

• Depth of Pearth tubes (inside eductor tube) at or near optimal

• The lances are well positioned to break up the core of the primary roll current

Figure 2: CFD results Velocity Vectors on Section through a Digester

Photograph 1: Helical Vanes on Eductor Tube

3.3 DIVE STUDY A dive study was carried out to validate the CFD study and confirm the effectiveness of the mixing system. The major findings of this study were:

• The primary flow pattern of a single toroidal roll current with its core at about two thirds of the tank radius inside the digesters is consistent with the results of the CFD analysis

• The fluid velocities and vector directions in the main body of the digester indicate an effective mixing pattern

• The lances are well positioned to break up the core of the primary roll current

• The near-floor and near-surface velocities were found to be less than those predicted in the CFD analysis, for the same gas flow

• Velocities in the central zones were also typically less than predicted, with the shape of the profiles similar to the CFD

• Testing of the near-floor velocities at three test points for three increased gas flows, gave only minor increases in velocity and variable trends

• Pumped mixing induces a significant flow rate inside the eductor tube, approximately five-times the pumping rate, and results gave an average fluid velocity inside the eductor tube of more than 20% that of the gas mixing value

• There appears to be no significant benefit to be gained by increasing the blower speed, which avoids wasting energy

• Time to achieve steady state mixing conditions, is in the order of hours

3.4 BLOWERS The existing digesters have positive displacement rotary lobe type blowers. It was decided that there is no compelling reason to have another type of blower for the new digesters, for what is a similar service, albeit with a greater flow requirement and a slightly greater submergence depth and hence greater pressure requirement. There are benefits in keeping a site standard blower (a spare of which is held on site) for digester gas applications. The blowers for the existing digesters have proven good long-term reliability with low maintenance. For the greater duty flow and pressure, the motor size and blower speed were increased (with a change of drive pulleys and belts).

The new blowers were located between the digesters at ground level thus improving access, reducing noise, and allowing one blower to serve either digesters.

One difficulty of thermophilic digestion is the greater biogas temperature, which necessitated cooling the biogas to maintain a temperature below 40°C into the blower. To achieve this, fins were added to the blower feed pipes (see Photograph 2), and in extreme conditions (high temperature, still air, mid summer, no cloud) water sprays in the biogas foam separator are set to automatically operate.

Photograph 2: Finned Biogas Pipe and Gas / Foam Separator on Feed to Blowers

4 FIXED COVERS

The existing mesophilic digesters at CWTP have floating covers that were the standard design in the 1950s and 60s. The floating covers have an annular gap to allow the cover to travel up and down and some odorous biogas is released from this gap. Because thermophilic operation can be more odorous than mesophilic, fixed covers are essential for thermophilic digesters and are now the standard for mesophilic digesters as well.

5 SLUDGE RHEOLOGY

At dry solids concentrations of greater than 2%, sludge exhibits non-Newtonian behaviour. To successfully design the sludge pumping system, it was necessary to determine the viscosity of the sludge. Thus, viscosity tests were carried out at the University of Canterbury. Some results of this testing are shown in Figure 3.

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160 180 200Shear Rate [s-1]

Shea

r St

ress

[Pa]

70/30 (Pri./Sec.) Mixed Sludge at 4.6% dry solids60/40 (Pri./Sec.) Mixed Sludge at 4.4% dry solidsPrimary Sludge at 4.6% dry solidsDigested Biosolids at 2.4% dry solidsWater

Figure 3: Results of Sludge Viscosity Testing at 20°C

A best fit of these results was used to determine the coefficients the Bingham fluid model (see Equations 1 and 3), which was used to determine pipeline pressure drops. The heat exchanger manufacturers used the Power Law model (see Equation 2).

γηττ &+= y (1) Where: τ is the shear stress, Pa yτ is the yield stress, Pa η is the coefficient of rigidity, Pa γ& is the shear rate, s-1

nKγτ &= (2) Where: K is the consistency index, kg/(m.s2-n) n is the power law exponent

γτµ&

= (3)

Where: µ is the viscosity, Pa.s

6 HEAT EXCHANGERS

6.1 GENERAL HEATING REQUIREMENTS Heat exchangers are required to heat the raw sludge and maintain thermophilic temperature (55°C) and to cool the partially digested sludge from thermophilic to mesophilic temperature (35°C). The cooling is achieved by pre-heating the raw sludge. Initial design also investigated recovering heat from the digested mesophilic sludge to the raw sludge. However this has a detrimental effect on the ability to cool the partially digested thermophilic sludge and without an alternative heat use, has little benefit. Thus the concept was not followed.

Options investigated for sludge heat exchangers (HXs) were as follows:

• Spiral heat exchangers – Rosenblad or Alfa Laval (two concentric spiral pathways with rectangular passage shape)

• Concentric tube (“tube-in-tube”) heat exchangers – various designs, including one with static mixers (multiple straight pathways with concentric circular and annular passage shapes)

• Tube and rectangular shell heat exchangers (“tube-in-channel”) – Läckeby design (multiple straight pathways with circular inner and rectangular outer passage shape)

The existing sludge HXs on site are of the spiral type. These are used for heating digested sludge re-circulated around the existing digesters, using hot water from the hot water “heat” loop. Hence, they are a water-to-sludge spiral design, and do not have the double opening doors of a sludge-to-sludge spiral HX, which limits the physical size of the units (for structural reasons). The “channel gap” (or passage width) of the existing HXs is 25 mm, and this is the same on both sides of the HX (water and sludge). Because the new sludge to sludge HXs will need to handle cold mixed raw sludge in the preheating stage, a channel gap of at least 25 mm was considered to be a key design requirement, to reduce the incidence of blockages.

For sludge-to-sludge applications, constraining the spiral HX design to a minimum channel gap of 25 mm would require approximately twice the number of HX units (connected in series or parallel). This would have required approximately 8 HXs, instead of the minimum number of 4 HXs required by the process (i.e. one for each duty). HX manufacturers could have offered the minimum number of units, but only by reducing the channel gap to within a range of 12 – 16 mm, and recommending sludge screening upstream of the HXs. After a study of this proposal, and a review of other projects with spiral HX used in this way, it was decided this was not an acceptable solution, and spiral HXs were not considered a viable option due to space required and having no cost advantages.

6.2 HEAT RECOVERY FROM SLUDGE COOLING After consideration of existing installations for these heat exchanger types, and comparison of equipment tendered by selected suppliers, it was decided that the most suitable option for this particular duty was the Tube-in-Channel type of heat exchanger, as offered by Läckeby Products Ltd, Sweden. This HX has a matched pair of water-to-sludge units connected by a closed-circuit water system.

The key factors in this decision, out of the many considered, were its specialised design for sludge applications, low frequency and ease of clean-outs, numerous directly relevant installations and a lengthy and satisfactory track record, as evidenced by operator comments.

6.3 DIGESTER HEATING Because this application is immediately downstream of the sludge-to-sludge HX, and receives raw sludge blended with digested sludge to varying degrees, it was considered important that this heat exchanger be the same or similar type, of HX as the sludge-to-sludge HXs. Hence, the Läckeby tube-in-channel HX was preferred.

7 SLUDGE PUMPS

The primary purpose of the sludge pumps is to draw sludge from the new buffer tank and digesters and either return the sludge for mixing purposes, or transfer the sludge to downstream digesters. Pumping to and from the digesters is via heat exchangers, for sludge heating and cooling purposes.

An important design aspect is the need for a chopper action at all sludge pumps, to minimise re-roping of hairs and other fibres to reduce blockages in heat exchangers and pipes, this being the outcome of an international peer review. Centrifugal chopper pumps with specialised impellers and an integral cutter bars have been developed specifically for digester applications.

As with the heat exchangers the main pumps were tendered separately and free issued to the contractor. Sludge pumps were required for the duties shown in Table 3.

Table 3: Sludge Pump Requirements

Pump Purpose Design Flow, l/s

Design Head, m H2O

Motor Rating, kW

Mixing the Buffer Tank 50 14.1 18.5 Feeding the Digesters 20.1 60 22 Circulating Sludge Through Heat Exchanger 27.8 25 30 Mixing the Digester (backup) 117 25 75 Transferring from Thermo. in Meso. Digester 20.1 37 15 Transferring from Meso Digester to Dewatering 20.1 28 11

All sludge pumps were originally intended to be centrifugal chopper pumps with a built in cutting bar. This type of pump reduces the size of solid particles in the sludge so that conventional solids-handling impellers can be used, rather than more open impellers with larger free passages. Vaughan was chosen as the preferred brand, being a specialised supplier with extensive experience in the field of digester applications, especially in regard to pumped re-circulation of sludge for mixing purposes. Vaughan chopper pumps have good reliability, with low maintenance, in New Zealand (notably at Mangere and North Shore WWTPs) and in numerous USA and UK installations.

However, for the higher head duties (greater than 25 m) the pump supplier Pump Systems Ltd offered Börger rotary lobe pumps instead of Vaughan centrifugal pumps, because Vaughan would have needed to use their maximum permissible speed and number of vanes for these duties, and they were not prepared to take a risk on pump performance and wear. Börger were able to keep the rotational speed low and included replaceable rubber lobe tips and casing liners in their rotary lobe pump, which are of a particular type specifically designed for high head solids-handling duties.

Comparing the two pump types, the rotary lobe pumps will have lesser energy cost (due to very high efficiency) but greater maintenance cost (due to lobe tip replacements) whereas the centrifugal pumps will have greater energy cost (due to medium-high efficiency) and lesser maintenance cost (due to infrequent maintenance).

The rate of wear is always an issue for solids-handling applications, but this has been addressed in two different ways for the two types of pumps; the centrifugal pump using Nickel-hardened casings and impellers at moderate speeds, and the rotary lobe pump using replaceable casing liners and rubber lobe tips at low speeds.

8 AUTOMATION

The design of the digesters was for a fully automated system with minimal operator time involvement. To achieve this, and obtain good operating data records, Table 4 shows the number of instruments and actuators required.

Table 4: Number of Instrument and Control Items

Item Number Variable Speed Drives 9 Electric Actuators 96 Flow Meters / Switches 6 Pressure Sensor / Switches 56 Temperature Sensors 33 Level Sensors 9 pH Sensors 2

9 SUMMARY

The design process has covered all aspects to considerable detail, reflecting the client’s need for energy efficient and reliable operation. This design is expected to continue the client’s experience of long intervals (15 years) between digesters being taken out of service for maintenance. The use of computational fluid dynamic (CFD) study and the verification of the CFD by a diver study in water, gives Christchurch City Council confidence in the expected mixing efficiency and effectiveness. The design enhancements in the mixing of these new digesters are expected to give even better mixing than is currently experienced in the existing mesophilic digesters on site.

The extra costs for thermophilic operation (mainly for heat exchangers and insulation) are expected to have a payback resulting from increased solids destruction with the resulting extra biogas for power generation and reduced polymer usage in dewatering plus reduced dewatered sludge cartage costs.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the Chemical and Process Engineering Department of the University of Canterbury for their assistance in sludge rheology testing and CFD analysis, and also Bernard Crossen and Niek Franssen (formerly with Christchurch City Council) for their involvement in the development of the design concepts.

REFERENCES

Dichtl N. (1997) ‘Thermophilic and Mesophilic (Two-Stage) Anaerobic Digestion’ Journal of the Chartered Institution of Water and Environmental Management, 11, 2, 98-104.

Oles, J., Dichtl, N. and Niehoff, H. (1997) ‘Full Scale Experience of Two Stage Thermophilic/Mesophilic Sludge Digestion’ Water Science and Technology, 36, 6, 449 – 456.