THE POTENTIAL FOR INCREASED RECLAMATION AND RE-USE OF EFFLUENT AT SAPPI ADAMAS

Authors

1Vinesh Dilsook, 1Maryna Mansfield, 1Henry Coppens, 2Andre van Niekerk and 1Volkmar Bhmer

Companies and addresses

1Sappi Technology Centre, Environmental Department, PO Box 3252, Springs, 1560, South Africa.
2Wates, Meiring & Barnard, PO Box 6001, Halfway House, 1685, South Africa.

emails

Vinesh.Dilsook@za.sappi.com, Henry.Coppens@za.sappi.com, maryna.mansfield@za.sappi.com, andrevn@wmb.co.za and volkmar.bohmer@za.sappi.com

Keywords

activated sludge, aluminium sulphate, brightness, closed systems, colour, effluent treatment, membrane bioreactor, nanofiltration, reclamation, re-use

ABSTRACT

To counteract the rising water and effluent tariffs, Sappi Adamas is considering increased re-use of its effluent.  Sappi Technology Centre investigated the potential for increased reclamation and re-use of effluent.  Biological treatment by activated sludge, followed by both chemical and physical colour removal methods, was investigated. Effluent treatment scenarios for increased reclamation and re-use of effluent were simulated. Activated sludge treatment followed by chemical colour removal was chosen as the option for increased re-use of effluent.  The proposed water and effluent management option will produce water acceptable for re-use in papermaking.  The potential return on capital investment is attractive.

1. INTRODUCTION

Increasing costs and environmental regulation has compelled mills to reduce both the volume and degree of contamination of their effluent discharges 1.  The reduction in process water consumption can be achieved by system closure. Systems closure is the progressive reduction in process water consumption.  In the process of closing up water systems, problems are caused by accumulates such as suspended solids, dissolved solids and thermal build-up 2.

As a result of escalating water and effluent discharge costs, mill management is considering increased reclamation and re-use of its effluent.  Adamas is a paper mill located in Port Elizabeth. The mill operates two paper machines, PM 3 (Kraft grades) and PM 4 (white and tint grades).  A variety of grades of paper are produced, hence the effluent composition is highly variable.

A simplified sketch of the current mill water system is shown in Figure 1.1.

Figure 1.1

Figure 1.1 Current mill water system

Adamas water system has the following features:

  • Potable water is used for dye and chemicals make-up and as a back up to the other water supplies.
  • Reclaimed sewage water is polished in a Dissolved Air Flotation unit (DAF 2) and used on PM 4.
  • The mill effluent is treated in DAF 1, where fibre is recovered. The clarified water is recycled to PM 3 and to the vacuum seal pumps of both the paper machines.
  • DAF 1 clarified water, not recycled to the Mill is discharged as effluent to the municipal sewer.

Sappi Technology Centre investigated the potential for increased reclamation and re-use of the mill effluent and proposed to rationalise the mill water system by the introduction of a biological kidney.

A consequence of water system closure is increased temperature due to the retention of thermal energy.  In the process of closing up a water system, the temperature at which bio-treatment can be conducted is a limiting factor.  The benefit of bio-treatment at increased temperature is that energy is conserved and cooling is not required 3.

The aims of this study were to:

  • Investigate biological treatment of DAF 1 product water at elevated temperatures (35oC 50oC) and
  • Investigate colour removal from the bio-treated water to produce water suitable for reuse.

2. MATERIALS AND METHODS

Activated sludge
Biodegradation studies were conducted using two 6 l temperature controlled bench-scale activated sludge reactors. Reactor 1 was the experimental reactor.  The temperature of Reactor 1 was progressively increased in 5oC increments from 35C to 50C. Reactor 2 was the control reactor and the temperature was maintained at 40C.

The feed comprised a composite sample of DAF1 product water. The sludge age was maintained at 8 days.  The hydraulic retention time in the activated sludge reactors was 6 hours. The performances of the activated sludge systems were monitored by measuring COD and colour removal.

Colour removal
Chemical (aluminium sulphate, poly aluminum chloride and ozone) and physical (activated carbon adsorption and nanofiltration) colour removal was investigated on bench scale units.  Colour removal was measured by spectrophotometric means 7.

Paper handsheets
Paper handsheets were made with the reclaimed biotreated water.  Handsheet properties were compared for ISO brightness (ISO 2470-1997).

3. RESULTS AND DISCUSSION

Activated sludge
COD removal by activated sludge treatment at elevated temperatures is summarised in figure 3.1.

Figure 3.1

Figure 3.1 COD removal by activated sludge treatment at varying temperatures

A significant drop in COD removal was observed as the temperature of Reactor 1 increased from 45C to 50C.  COD removal greater than 75% was achieved with Reactor 2. The COD removal by Reactor 2 steadily increased as the micro-organisms acclimatised to the temperature and the effluent feed.

In this study the maximum temperature for acceptable COD removal by activated sludge treatment proved to be 45C. The possible reasons for the activated sludge system failing at the high temperatures were stresses encountered by the bacterial sludge caused either by the increased temperature or reduced dissolved oxygen concentration at the higher temperatures.

Colour removal
The feed and the product water of the bioreactors were measured for colour.  Colour removal by activated sludge was variable.

Irrespective of the residual dye colour in the feed effluent, a consistent pale-yellow colour was observed in the biotreated product water.  This indicates the micro-organisms are effective in absorbing, metabolising and/or transforming the residual dye in the effluent.

The pale-yellow colour in the biotreated water must be removed before the water is re-used.  With the exception of aluminium sulphate and nanofiltration, all other methods investigated to removal colour from the biotreated effluent were unsuccessful.

Effluent treatment scenarios
Based on COD reduction and colour removal, three effluent treatment scenarios for increased reclamation and re-use of effluent were simulated. These scenarios are summarised in table 3.1.

Table 3.1 Reclamation and re-use scenarios

Option A

Option B

Option C

DAF 1

DAF 1

DAF 1

Activated Sludge

Activated Sludge

Membrane Bioreactor (MBR)

Chemical colour removal (aluminium sulphate) in DAF 2

Membrane colour removal (Nanofiltration)

Membrane colour removal (Nanofiltration)

Re-use

Re-use

Re-use

Payback: A years

Payback: A x 2.6 years

Payback: A x 3.2 years

These scenarios were compared on a techno-economic basis (table 3.1). Option A was found to be most suitable 4.  Figure 3.2 represents a schematic of option A.

Figure 3.2

Figure 3.2 Water and effluent management option A

Option A involves the following key components:

  • Collection and pre-treatment of effluent in DAF 1.
  • Recycle of DAF 1 product water to the applications not requiring high quality water, including PM 3 and Vacuum Seal Pumps.
  • Blending of reclaimed sewage water with biotreated water.  Polishing of the blended water in DAF 2 (using aluminium sulphate). This will supply high quality water to PM 4.
  • The recycled water to the Vacuum Seal Pumps must be cooled to ensure optimum operation.
  • A small amount of mill effluent will be discharged to purge the water system and to control salinity build-up.

However, certain aspects of the proposed effluent management programme required further investigation. A second investigation was conducted to:

  • Establish the cause of poor performance by the activated sludge system at elevated temperatures and
  • To determine the effect of the reclaimed water on paper properties.

Oxygen activated sludge
The poor performance by activated sludge at 50C was attributed to either the effect of the high temperatures on the micro-organisms or the reduced oxygen solubility. Oxygen supplemented activated sludge treatment was hence investigated.  Oxygen supplemented activated sludge conditions were achieved by blending pure oxygen with the compressed air used to aerate the system.

The results of oxygen activated sludge treatment of the mill effluent are summarised in table 3.2.

Table 3.2 Average COD values of oxygen supplemented activated sludge treatment

 

Temperature

Sample size

COD (mg/l)

COD Removal (%)

Feed

Product

 

 

 

 

 

 

Reactor 1

50C

35

1315

487

63

 

 

 

 

 

 

Reactor 2

40C

35

1315

244

81

 

 

 

 

 

 

Inconsistent COD removal was observed at 50C. Consistent COD removal was observed at 40C (table 3.2). An average COD removal of 63% was achieved at 50C compared to the previous finding of 49%. The small improvement in COD removal efficiency is attributed to the use of oxygen.

Chemical colour removal (aluminium sulphate)
The proposed treatment, option A keeps the water system partially open.  The make-up water is reclaimed sewage water (SW).  Polishing of the blended activated sludge product water and SW water was investigated.

Three blends of reclaimed water were treated with aluminium sulphate:

  • 100% biotreated water (AS),
  • 75% bio-treated + 25% SW and
  • 50% bio-treated + 50% SW.

Figure 3.3

Figure 3.3 Colour removal from blended activated sludge treated water using aluminium sulphate

Optimal colour removal from the reclaimed water was observed at an aluminium sulphate dosage of 300 mg/l (figure 3.3). The caustic required for neutralisation is in the order 1214 mg/l.

Paper handsheets
Laboratory handsheets were made with the present DAF2 product water and the reclaimed water. The properties of the handsheets were compared. Handsheet brightness using polished reclaimed water is illustrated in figures 3.4 and 3.5.

Figure 3.4

Figure 3.4 Comparison of handsheet brightness using recycled water treated with aluminium sulphate (100 mg/l)

Figure 3.5

Figure 3.5 Comparison of handsheet brightness using recycled water treated with aluminium sulphate (300 mg/l)

Brightness of handsheets produced is influenced by quality of the recycled water used. Handsheets made using the 50% biotreated water blend had the least effect on brightness loss (figure 3.5).  

4. CONCLUSION

Activated sludge treatment of the mill effluent was restricted to 45C.  The performance of activated sludge treatment is poor at 50C because of temperature-related stresses on the micro-organisms and not a lack of soluble oxygen. 

If the water system of Adamas is closed, the temperature should be maintained at 45C. The optimum aluminium sulphate dosage for polishing of the reclaimed water is 300 mg/l (figure 3.2).

There was a small loss in paper brightness using reclaimed water.  The brightness loss can be corrected by a small increase in OBA addition (approximately 1 kg OBA per ton of pulp for every 1 unit drop in brightness) 5

The proposed water and effluent management strategy will:

  • Reduce the mill dependence on external water sources,
  • Provide protection against future water and effluent rate increases, and
  • Produce water suitable for re-use in papermaking.

Sappi has several minimum impact mill projects on the cards featuring treated water re-use on the paper machines. Implementation at Adamas will provide a learning experience and raise confidence to implement similar projects with reduced environmental impact and greater savings at other mills.

REFERENCES

1. Curley, J., Jones, B. and Wiseman, N. 1999.  Closing the loopreducing the reliance on fresh water and minimising drain losses; some new developments.  TAPPI International Environmental Conference, pp. 995-1010.

2. Ramaekers, J.P.C., van Dijk, L., Lumpe, C., Verstraeten, E. and Joore, L. 2001.  Thermophilic Membrane Bioreactors in the Pulp and Paper Industry A successful key to in-mill water treatment. Paper Technology.  February 2001 p. 32-40.

3. Tripathi, C.S. and Allen, DG. 1998. Feasibility study of thermophilic aerobic biological treatment of bleached Kraft pulp mill effluent. TAPPI International Environmental Conference, pp. 1189-1201.

4. Van Niekerk, A. 2001b. Water and effluent management report. Wastes, Meiring and Barnard, report: 4624/2452/3/P

5. Thomson, N. [Clariant (Pty) Ltd], 2001, personal communication.

6. Dilsook, V. 2001a. Bench-scale studies at Sappi Adamas. Sappi report: R&D2001/008E.

7. Standard Methods for the Examination of Water and Wastewater. 1998.  20th Ed. p 2.3 -2.6.

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