Purification of landfill Leachate with Membrane Filtration

T he most economical and environmentally friendly way to treat

landfill leachate is to reduce its volume by 75 to 80% using reverse osmosis and then return the concentrate to the landfill by controlled reinjection. If this procedure is not yet authorized by local authorities then the treatment process must achieve very high rates of recovery by using a combi- nation of reverse osmosis (RO) and nanofiltration DT- module technology with controlled crystallization. Dr.-lng. Thomas A. Peters discusses.

INTRODUCTION

Toxic and hazardous com- pounds can originate from landfill leachate as a result of the soluble components of solid and liquid wastes being leached into surface and ground water. So land- fill leachate is comparable to complex industrial waste streams which contain both toxic organic and inorganic contaminants.

An evaluation of the data for the leachate from more than 150 landfills (Dahm, 1994) shows, that the amount of components dissolved in leachate from different kinds of landfills covers the range from 2 to 15 g/l. From this the fraction of organic components is considered to cover a range between 0.1 and 3 g/l. That is much smaller than the inorganic part, indicated to have a range from 1.6 to 14.3 g/l, including ammonia with values between 0.3 to 2 g/l.

Since such contami- nants are mostly not appro- priate for treatment by conventional biological processes new regulations tend to limit the discharge of such complex wastes to municipal sewers. Even by combining biological treat- ment with adsorption by active carbon or with the

oxidation of part of the dissolved organic material using ozone or other oxidizing agents, only partial destruction of conta- minants will be achieved. It will not reach the purifica- tion needed to fully reduce the negative impact of landfill leachate on the environment (Peters, 1996). One aspect is the so called “hard COD” that is not biodegradable and can- not be destroyed or adsorbed and thus will remain in the water dis- charged after being treated with the mentioned processes, causing prob- lems in the future as a con- sequence of the effects of long time accumulation.

Therefore more effec- tive methods of treatment of this material have had to be developed. The use of reverse osmosis either as a main step in a landfill leachate treatment chain or as a single step has shown to be a very successful means of achieving full purification.

RO FOR THE PURIFICATION OF LAND-

FILL LEACHATE

Due to the ability of modern high-rejection reverse osmosis membranes to retain both organic and inorganic contaminants dissolved in water at rejec- tion rates of 98-99%, reverse osmosis is also useful for purifying liquid waste such as landfill leachate and for helping to solve the growing problem of water pollution.

On the feed-water side of the membrane, the dissolved organic and inorganic contaminants are concentrated in the retentate, whereas the pure water is “pressed” through the membrane. Consequently with the reverse osmosis mem- brane a treated water stream - the permeate - is

generated, which contains only very low levels of inor- ganic and organic contami- nants. These meet potable water standards and dis- charge of this water to the next river or aquifer con- tributes to maintenance of the natural equilibrium as this leachate was originally mainly clean rain water.

As the reverse osmosis membrane operates like a well-defined barrier the purification process itself can be controlled continu- ously and with a high degree of security by the simple and precise mea- surement of the electric conductivity.

Furthermore the mem- brane process of reverse osmosis offers a high oper- ating stability, as the start- up and shut-down of the respective plants is switch- operated and carried out in few minutes. Also the stand- by of such systems for short or even long periods of time are easy to handle, as the purification process is acti- vated at least only by a pressure produced with pumps.

Because of the high rejection rate for each kind of contaminant dissolved in the feed water a high flexi- bility against changes of the concentration of the com- pounds in landfill leachate is given. Therefore the perme- ate produced has always the expected high quality, as this is based on a repro-

ducible high purification efficiency.

The same flexibility can be stated for changes in the volume to be treated. The modular design of reverse osmosis plants allows for a quick increase or decrease of the purification capacity by adding or taking away membrane area.

However in addition to requiring highly resistant membranes, the treatment of landfill leachate with reverse osmosis demands the use of open channel module systems that can be cleaned with high efficiency with regard to scaling, foul- ing and especially biofoul- ing. Therefore tubular mod- ules were the first medium used in the early reverse osmosis systems for the purification of landfill leachate starting in 1984. An alternative was intro- duced to this market in 1988. The disc-tube-module (DT-module) has been installed since then with great success (Peters, 1995). By the year 1997, plants equipped with this DT-module represented more than 75% of the total capacity installed for the purification of land- fill leachate by reverse osmosis.

The DT-module using selected membranes and a special plant design can meet further requirements expected from customers in this application including

TABLE 1: TYPICAL PLANT PERFORMANCE IN LEACHATE PURIFICATION

parameter leachate permeate I permeate II rejection pH-value 7.7 6.8 6.6 (in %) el. conduct.* 17 250 382 20 99.9 COD in mg/l 1797 < 15 < 15 > 99.2 ammonium mgO,/l 366 9.8 0.66 99.9 chloride mgil 2830 48.4 1.9 99.9 sodium mg/l 4180 55.9 2.5 99.9 heavy metals I# 0.25 < 0.005 < 0.005 > 98

* = pS/cm # = mg/l

easy handling, low energy consumption and optimized operation costs. This is proved by more than 100 plants that are in continuous operation - for many years and under rough working conditions - on landfill sites in Europe, North America and Far East.

The successful opera- tion of reverse osmosis in the plant of the municipal waste landfill of lhlenberg (former VEB Deponie Schonberg) near the city of Ltibeck in Germany, the most modern and largest multi-stage plant that has been realized up to this time for landfill leachate purifica- tion; demonstrates the pos- sibilities of modern mem- brane technology. Some results are given as exam- ple in Table 1.

This reverse osmosis system with a capacity of 36 m3/h has been in opera- tion without any problem since December 1989 with one change of the mem- branes till now. With two reverse osmosis stages, the average rejection rates for salts and organic contami- nants are about 99%. Depending .on the salt con- tent of the feed water and the operation time between the cleaning cycles, the operating pressure ranges between 36 and 60 bar at ambient temperature. The specific permeate flux was calculated to be approxi- mately 15 l/m*h.

Similar results are reported from other plants, for example from the landfill Kolenfeld near Hannover, with start up in February 1990. This plant, oper- ated with 1.8 m3/h, has an availability of over 90%. The electric conduc- tivity of the feed is repor- ted to be 15 000 to 16 000 pS/cm, the rejection rate always more than 98%, for COD 99%. New mem- branes were installed in April 1993, after more than 3 years in operation because of a decreasing permeate flux.

These data from long- term experience have also been confirmed by the results of the other 100 or so systems mentioned above that are in operation on different landfills up to now and by the data col-

lected during numerous tests with pilot plants in technical scale all over Europe, North America and in some countries in Far East.

The results prove that reverse osmosis is a very effective instrument for the purification of landfill leachate, if all design crite- ria and requirements specif- ic for landfill leachate have been taken into considera- tion and an adapted module system is used.

VOLUME REDUCTION FOR THE CONCENTRATE

Steady improvement of membrane technology has resulted in a high pressure reverse osmosis system based on the DT-module with operation pressures in the range of 120 bar and an adapted process to reduce certain salt fractions by con- trolled precipitation. High pressure modules are con- nected to a system of tanks where crystals of, for exam- ple, CaSO, precipitate and are discharged continuous- ly from the bottom of the tanks as sludge. In some instances, depending on the leachate, seeding nuclei may be added to promote crystallisation.

With these develop- ments, the limits for the recovery rate in landfill leachate - given among oth- ers by the osmotic pressure - have been overcome and the concentration factor for the organic and inorganic matter dissolved in the land- fill leachate was doubled. This means an increase of the permeate recovery from about 80% - related to a concentration factor of 5 - to 90% recovery with a con- centration factor of 10 for the contaminants retained by the membrane. Thus the limit for the electric conduc- tivity in the concentrate of a reverse osmosis plant was increased from 50 000 to 60 000 us/cm (which is the limit of usual reverse osmo- sis systems) to the range of 100 000 to 120 000 uS/cm (Peters, 1995):

Due to the high volume reduction related to this increase of pure. water recovery, this technology allows the need for subse- quent evaporation steps to

be eliminated. After being processed with high pres- sure reverse osmosis, the concentrate can then be fed directly into a dryer or a solidification device, or be burned. Such high pressure reverse osmosis systems are in use actually at about 25 plants.

The high pressure oper- ation opens new possibili- ties for the solution of sepa- ration problems in chemical engineering. In combination with seeding techniques, it can be used for further improvement of the perme- ate recovery. For example, the problems associated with the solubility of high concentrations of calcium sulfate in one particular leachate are overcome by a controlled crystallisation. The crystals are separated from the concentrate - low- ering the scaling potential and supporting thus a fur- ther concentration of the retentate - and deposited without problems on the landfill. With this procedure a permeate recovery of up to 95%, equivalent to a con- centration factor of 20, can

be made possible* under certain circumstances., These have to be very well studied in each case to find the limiting factors and to be able to design the right solution.

The high pressure reverse osmosis makes necessary slight changes in operating reverse osmo- sis systems, but shows the same reliability and repeatability of good results as the DT-module in the standard configura- tion till 65 bar operating pressure.

On the lhlenberg landfill site high pressure reverse osmosis has been working since January 1992 with a salt rejection of more than 99%, for example from 73 280 @/cm in the feed to 434 @/cm in the permeate. The average specific ener- gy demand for this concen- trate stage is 14 kWh per permeate, whereas the leachate stage with 80% recovery in front of this sys- tem consumes less than 5 kWh/. The continuous growth of this landfill and the accompanying increase

TABLE 2: EXAMPLE FOR REJECTION RATES FOR NANOFILTRATION WITH DTF-MODULE

Deponie lhlenberg

parameter unit feed permeate rejection (“4

BOD, Cm9 0,/I) 480 280 41.62 COD (w 0,/l) 17000 700 95.88 ammonia (w/l) 3350 1420 57.61 sulphate (mdl) 31200 2345 92.48 chloride (w(l) 12760 17730 -38.95 calcium (w/l) 2670 187 93.00 magnesium (w/l) 1030 72.7 92.94 sodium (ms/l) 10900 5010 54.04 pH-value __ 6.3 6.4 elec.conductivity (uS/cm) 61 43 29.5

Deponie Halle-Lochau

parameter unit feed permeate rejection (%I

BOD, (ms 02/1) 591.4 239.6 59.49 COD (ms 0,/I ) 6400 600 90.63 ammonia (w/l) 2260 1020 54.87 chloride (w/l) 29780 35629 -19.64 calcium (w/l) 243 31 87.24 magnesium (w/l) 952 133 86.03 sodium (w/l) 41000 20390 50.27 pH-value @w/l) 6.23 6.31 elec. conductivity (uS/cm) 120 100 20

Figure 1: Nanofiltration plant for 5.4m3/h feed

Figure 2: Combination of high pressure reverse osmosis and nanofiltration

in leachate required the expansion of the purification capacity. In December 1993 the next reverse osmosis plant was brought online with an integrated high- pressure stage that increased the input capacity to another 48 per hour of raw leachate (Kenner, 1995).

In some cases a DT- module for an operating pressure of 200 bar is used behind the 120 bar stage to achieve the high permeate recovery rate of up to 95%. One exception is the landfill site of Halle- Lochau, where 17 m3/h land-fill leachate are processed with an overall recovery of 87 to 88%, actually up to 90%. This here is the limit as the untreated leachate has already a electric conduc- tivity of 40 000 uS/cm (Rapthel, 1994). The avail- ability of this system, that has been in continuous operation since February 1993, is more than 95%, the permeate leaving the plant has nearly the same

quality as that reported for Ihlenberg.

PROCESS IMPROVEMENT WITH

NANOFILTRATION

Even better permeate recovery rates can be reached by installing a combination of nanofiltra- tion and fractionated removal of solids together with reverse osmosis and high pressure reverse osmosis. Nanofiltration allows material dissolved in water to be separated into monovalent and bivalent ions. Consequently, the high rejection rate for sul- phate ions and for dis- solved organic matter together with very low rejection for chloride and sodium reduces the vol- ume of concentrate. Some examples for rejection rates for different compo- nents dissolved in landfill leachate are shown in Table 2 (Rautenbach, 1995).

For this application, very specific nanofiltration

membranes have to be selected and the module must be suitably designed to optimize the interaction of flow parameters such as feed flow velocity, pressure drop, efficient membrane cleaning, insensitivity to micro-particles. Also, a good cost/performance ratio must be achieved. These requirements for successful and continuous separation of heavily loaded waste water are met by ROCHEM’s DTF- module, a flat channel module consisting of only a few components in which open-channel construction is again combined with nar- row gap technique.

A system operated with 8.5 kWh per m3 of totally produced permeate with a water recovery rate of 97% is one example of the extremely low overall power consumption of this hybrid process using reverse osmosis, nanofiltration and fractionated removal of solids.

Figure 1 shows a nanofiltration plant in opera- tion in Halle-Lochau since September 1994, Figure 2 the combination of high pressure reverse osmosis and nanofiltration.

The fact that a permeate recovery rate of 95 to 97.5% is operating standard today shows that the combination of reverse osmosis with nanofiltration and crystalli- sation (Figures 3 and 4) is the basis for a economical process for the purification of landfill leachate.

Based on the technolo- gy explained above, com- panies have introduced an “own and operate” service, where clients simply pay a price per m3 of leachate treated without capital risk and with minimal opera- tional involvement. Beside this usual way of handling the leachate purification on a landfill site ROCHEM decided additionally to operate its equipment for short term emergency situa- tions, caused by heavy pre- cipitation and capacity bot- tle-necks.

One example is a former disposal facility for liquid waste from the petroleum industry, near Houston, USA. The leachate of the bioremediated sludge of

this superfund site was processed with two DT-sys- terns with 27 m3/h feed capacity each. Examples for the quality reached: the TDS was reduced in single pass from 2500 to 8 mg/l, TOC from 1750 to 4 mg/l and TOX from 155 to 0.002 mg/l. The permeate, meet- ing National Pollution Discharge Environmental Standards, was discharged directly to a river.

HANDLING OF LEACHATE

CONCENTRATE

The purification of landfill leachate with membrane technology helps to avoid further contamination of the resources like groundwater and surface water. But beside the ecological aspect - the minimisation of the burden on the environ- ment - the commercial fea- sibility i.e. the affordability also has to be taken into consideration. In this regard membrane filtration has proved to be a justifiable and economic solution in most of the cases, even when the overall costs for the purification are com- pared with other approach- es for the treatment of land- fill leachate.

This evaluation includes the handling of the concen- trate produced in the reverse osmosis plants, that in the past has been consid- ered to be a very cost inten- sive step. That applies in fact for few plants in opera- tion, that are using evapora- tion and drying followed by deposition of the dry residues in a special landfill.

However, today other possibilities are gaining importance. They have been developed consider- ing the best technology available and at the same time ecological and eco- nomical requirements. These are:

A) transport of the con- centrate to an incineration plant equipped for the burn- ing of liquid hazardous waste;

B) the solidification of the concentrate with differ- ent materials, like fly ash (Kenner, 1995) or sludges from waste water treatment plants (Rapthel, 1994), and disposal of this kind of dry

landfill leachate

RO concentrate

igure 3: Diagram for the combination of reverse osmosis, high pressure reverse osmosis and nanofiltration

25 to 28 % with-

concentrate discharge 2to546

:igure 4: Illustration of the high recovery rate obtained with combination process

residue on the landfill itself. If these solids are produced under controlled conditions no further pollution is to be expected;

C) controlled reinjection of the concentrate into changing areas of the land- fill in order to improve the biochemical degrada- tion process in the waste itself and accelerate the immobilisation of the organ- ic matter.

This solution should be

approach as isandaill ,‘:I? considered

ally can be compared to a bioreactor that under opti- mal operating conditions will produce valuable landfill gas, in this case accelerat- ing at the same time the desired immobilisation of organic components (Pohland, 1996).

If the system for the controlled return of the concentrate to the landfill is designed according to the

needs and the respective conditions on site no changes for the concentra- tion of pollutants in the leachate being pumped from the landfill to the purification plants are to be expected. This has been demonstrated on different landfills, in one case over a period of more than ten years (Grosse, 1990; Dahm, 1994; Henigin, 1995).

Nevertheless the deci- sion about the concentra- tion factor of a leachate purification system should be based on evaluation cri- teria that include all kind of economical and ecological aspects specific for each landfill site in order to find an optimized solution. In some cases a one stage unit could be sufficient, in others a combination of processes with the highest possible recovery rate should be selected.

CONCLUSIONS

The results obtained during the operation of an increas- ing number of plants under very different conditions prove that reverse osmosis is a very effective instru- ment for the purification of landfill leachate if all design criteria and requirements specific for landfill leachate have been taken into con- sideration, and if an adapt- ed module system as well as correlated technologies are used.

This includes high pres- sure reverse osmosis, with operating pressure up to 120 bar and nanofiltration in combination with a controlled crystallisation process, that allows perme- ate recovery rates of more than 97%.

The elimination of the negative impact of landfill leachate on the environ- ment can be achieved with

membrane filtration% due to the dramatic minimiza- tion of residual waste to be processed or immobi- lized and due to the high quality of the purified water discharged back to nature. The combination of processes designed for this purpose is one example of a sustainable environmentally friendly development.

REFERENCES

Dahm W., Kollbach J. St & Gebel J. (1994) Sickerwasser-Reinigung _ Stand der Technik 1993/94 - zukiin- ftige Entwicklungen. EF-Verlag fiir Energie- und Umwelttechnik GmbH, Berlin. Grosse G. (1990) Mehrjlhrige Betriebserfahrungen durch Einsatr des Umkehrosmoseverfahrens und RijcMiihrung von Konzentraten auf der Deponie. Lehrgang ,,Deponiesicketwasseraufbereitung bis zur Direkteinleitung und Totalentsorgung“, TAW, 30.131 .Ol .I 990, Ostfildern. Henigin P.L.A. (1995) Recirculation of Leachate Concentrate from Reverse Osmosis Treatment. Proceedings SARDINIA 95, Fifth International Landfill Symposium, CISA, 2.-6.10.1995, S. Margherita di Pula - Cagliari, Italy. Kenner A. & Peters Th. (1995) Sickerwasser-Reinigung auf der Deponie Ihlenberg. WLB Wasser, Luft und Boden T-211995, pp. 24-27. Peters Th. (1995) Volume Reduction of Leachate Concentrates with Modules for Nanofiltration and Reverse Osmosis. Proceedings SARDINIA 95, Fifth International Landfill Symposium, CISA, 2.-6.10.1995, S. Margherita die Pula Cagliari, Italy. Peters Th. (1996) Purification of Landfill Leachate with Membrane Technology. WATER QUALITY INTERNATIONAL, 9.-10./1996, pp. 23-26. Pohland F. (1996) Landfill Bioreactors: Fundamentals and Practice. WATER QUALITY INTER- NATIONAL, 9.-l O./l 996, pp. 18-22. Rapthel M., Pohl W. & Peters Th. (1994). Investigations regarding the purification of landfill leachate at the landfill site Halle-Lochau. Lecture abstracts, ACHEMA 1994, Frankfurt. Rautenbach R. & Linn T. (1995) Aufarbeitung von Konzentraten aus der Deponiesickerwasser- Behandlung mittels Nanofiltration. ENTSORGUNGSPRAXIS, 9/91, pp. 44-48.

ACKNOWLEDGEMENT

This article was published courtesy of CISA. First pub- lished in the proceedings of the Sixth International Landfill Symposium, 13-17 October, 1997, Sardinia, Italy.