CHEMOX™: Advanced waste water treatment with the impinging zone reactor

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Pergamon

Waf. Sci. Tech. Vol. 35, No.4, pp. 347-352,1997.

PH: S0273-1223(97)00044-9

© 1997 IAWQ. Published by Elsevier Science Ltd Printed in Great Britain. 0273-1223/97 $17'00 + (}oo

CHEMOXTM: ADVANCED WASTE WATER TREATMENT WITH THE IMPINGING ZONE REACTOR Peter A. Barratt*, Arne Baumgard*, Neil Hannay*, Manfred Vetter* * and Feng Xiong* * European Technology Group, Air Products Pic, Crockford Lane, Chineham, Basingstoke, Hampshire, RG248FE, UK ** Rhein-Sieg-Abfallwirtschafts-Gesellschaft mbH, Pleiser Heeke 4, 53721 Siegburg, Germany

ABSTRACT Air Products uses their CHEMOXTM process for advanced oxidation of a variety of waste waters, e.g. landfill leachate. The centrepiece of the CHEMOXTM process is the Impinging Zone Reactor (IZR) developed by the University of Clausthal. In the IZR waste water is intensively mixed with an ozone/oxygen mixture under atmospheric pressure and ambient temperature. Due to the excellent gas mass transfer performance of the IZR no expensive pressure vessels or gas compressors are required. Pilot plant trials were performed very successfully on a landfill leachate in Germany in 1995. Results from parallel operations with a commercial venturilbubble column system and the CHEMOXTM process show a decrease in hydraulic retention time of 10.3 vs. 3.3 [h], a reduction of the 03/COD ratio of 3.2 vs 1.8 [kglkg], and an energy consumption (without ozone generation) of 8.0 vs. 4.3 [kWhlkg ozone consumed] (meter reading) respectively. State-of-the-art, process controlled, mobile pilot plants provide the customer with the required information to accurately design and cost an effective ozone waste water treatment process. © 1997 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Advanced oxidation; CHEMOXTM oxygen; water.

environmental; impinging zone reactor; landfill leachate; ozone;

INTRODUCTION Due to various reasons there are continuously increasing requests for new waste water treatment applications for non-biodegradable substances in water. The favoured oxidant for many of these applications is ozone. Hereby, the customers usually request a compact system which: reduces the COD (Chemical Oxygen Demand) and AOX (Adsorbable Organic Halides) concentrations below the legal limits, uses only a minimum of ozone (oxygen), keeps the energy costs low, is safe to operate. 347

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Many chemical reactions in waste water are fast and require there~ore high ozone concent~ations. The ~~~ & f . t water , measured as the liquid side volumetrIc mass transfer coefficIent, KLa. [h-' ] l'. transler 0 ozone In 0 . the limiting step in many ozone treatment systems. To handle this problem conventIOnally, either thf retention time in the reactor has to be increased (large bubble columns) or a pressure of sever~ atmosphere! has to be applied. Both solutions are capital intensive and pressure syste~s are usually aVOIded whenevel 'bl (safety risk' compression energy costs). Jet loop reactors proved In the past to be able to overcome pOSSI e , . . h' h K al b some of these difficulties. They are usually low pressure systems~ aChIeVIng. Ig .La v ues .y producing large amounts of small bubbles. The draw back of this system IS the relatIvely hIgh P?wer Input for the recycle pumps. Air Products' European Technology Group (ETG) has now o:ercome thI~ problem ~ith its recently developed CHEMOXTM process. It combines two jet loop reactors Into one umt and proVIdes an outer vessel in which slower chemical reactions take place. METHODS The CHEMOXTM process is an advanced oxidation technology to treat a variety of non-biodegradable substances in water, e.g. landfill leachates and colours. The waste water is mixed with a combination of ozone, hydrogen peroxide and UV-light in an unique new reactor, achieving high mass transfer rates and low energy consumption simultaneously. This new reactor, the Impinging Zone Reactor (IZR), was initially developed by the University of Clausthal in Germany and is based upon some of the principles of a jet loop reactor. The operating principle of the IZR is described extensively in the literature (Vogelpohland Gaddis, 1993) and Figure 1 shows a schematic of the pilot plant. Oxygen is supplied from a liquid oxygen storage tank outside the building and gaseous oxygen fed to the ozone generator (150 [g 03]1h max. output ). From there the ozone/oxygen gas mixture flows by gas pressure only to the nozzles of the IZR. There it is mixed with water which is pumped at a high velocity in a recycle loop. Two of these liquid-gas streams are then impinged in a centre tube where they create a highly turbulent zone. The centre tube is interconnected with a reservoir to allow the dissolved gas more time for chemical reaction with species in the liquid (waste water), before the treated liquid leaves the vessel at the top. Additional waste water is pumped continuously into the recycle loop. Hydrogen peroxide is fed optionally by another pump into the same loop. UV-light treatment can also be applied to the water. The off-gas leaves the vessel at the top through an ozone destruction catalyst and then to atmosphere. Ozone concentrations were measured with a UV absorbance detection in the gas feed and in the off-gas from the reactor. The IZR used for the first pilot plant was a unit of 200 litre total volume made of PVc. The PVC construction provided us with flexibility for possible changes in the design. After initial trials on colour removal in the UK (Barratt abd Xiong, 1995), the pilot plant was installed in October 1994 at St. Augustin landfill, Germany, where the owner and operator, the Rhein-Sieg Abfallwirtschaftsgesellschaft (RSAG), installed a commercial ozone landfill leachate treatment plant 3 years ago. Landfill leachate is one of the most difficult waste waters to treat, due to the complex nature of the COD components. This commercial size plant treats landfill leachate from the local and other municipal waste dumps with a combination two stage biological treatment, three stage chemical treatment using ozone in a venturilbubble column system and a second biological plant at the end of the process. The plant has been required to meet the tough local ~erman regulations of maximum COD «300 [mg/I]) and AOX «500 [Jlg/l)) concentrations for waste water dISPOSal. The chemical oxidation step performed at this plant was simultaneously performed by the Air Products pilot plant. In addition t~ ~he landfill leachate produced at St. Augustin, a second biologically treated leachate from an~th~r ~umc~pal landfill was imported via road tanker and trials run with the pilot plant. Similar optIffilsatlOn ~nals were performed on both effluents so that results could be compared. The effluent produced on SIte (St. ~ugustin) is referred to as leachate A and that imported from another site as leachate B. T.able ~ shows the typIcal concentrations of principal chemical components of the two landfillleachates after bIOlogIcal pretreatment.

Advanced waste water treatment

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Table 1. Chemical characteristics of two landfillleachates COD[m~1l] BOD[m~1l]

AOX [Ilgn] Total Nitro~en [mgll]

,Leachate A 460 8.4 360 370

Leachate B 1070 14.4 720 67

The ranges of each critical process parameter used during the trials are given in Table 2. Table 2. Range of variable trial process parameters Range Parameter 3 0.4 - 0.75 Gas flow rate [m /h] 1-5 Liquid feed flow rate CUmin] 70-120 Ozone ~as feed concentration [m~] 1.2* Pressure in IZR [bar~] 10-30 Temperature rOC] 3-10 Period of trial run [h] (*: Due to catalytic ozone converter m off-gas lme) The trials were performed in the following way. The IZR was filled with biologically pretreated leachate and the recycle pump, feed pump and ozone feed flow were started. The ozone inlet and outlet concentration of the IZR as well as gas and leachate flow rates were logged into a computer on-line. After around 3-4 [h] the pilot plant reached steady state. Samples of treated and untreated effluent were taken regularly during the course of a run and analysed for COD using the standard dichromate oxidation method (Clesceri, 1989). The 03/COD ratio was calculated on the basis of the mass of ozone applied to the process and the mass of COD

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amount of ozone fed to the IZR, although a part of it left the IZR again in the off-gas. The Hydrauli4 Retention Time (HRT) was calculated on the basis of a reactor volume of 200 [1]. The energy input require< to transfer the ozone into the liquid phase was directly measured with a power meter as the electrical powel used by the recycle pump. RESULTS The main results gained from the leachate treatment trials were: the 03/COD ratio is mean 1.8, low 1.5; this is displayed in Figure 2; the 03/COD ratio is based on all the 03 added to the IZR; significant amounts of 03 were in the off-gas; the addition of H20 2 does not improve the performance; this is displayed in Figures 2 and 3; the achieved COD output levels are acceptable for discharge into the municipal sewage system in Germany; the required HRTs are less than 4 [h]; this is displayed in Figure 4; the CHEMOXTM 03/COD ratios were always lower than the 03/COD ratios of the RSAG plant (average value: 3.2). Table 3 summarises the results and provides a comparison between the CHEMOXTM process and the venturi/bubble column system of RSAG. The power represents that required to transfer the ozone from gas to liquid phase. Note particularly the differences in the HRT. Table 3. Leachate A - Comparison of performance of two ozonation processes

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Venturi I Bubble Column System

Impinging Zone Reactor (CHEMOXTM)

3.2 01/COD [gig] HRT [h] 10.3 COD @ inlet [mgll] 460* COD @ outlet [mgll] <150 COD removal [%] 70 (approximately) Power [kWh/kg 0 1 ] 8.0 (*: mean of 17 mdependent tnals data sets)

1.8* 3.3 460* 131 * 71* 4.3

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Advanced waste water treatment

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Figure 4. Relationship between retention time and COD removal.

Table 4 shows the mean data collected with the optimised CHEMOXTM process on leachate B.

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Table 4. Leachate B - Perfonnance of the CHEMOXTM process 1.7* 01/COD [gig] 4.2* HRT [h] 1050 COD @ inlet [mg/l] 343 COD @ outlet [mgll] 67* COD removal [%] 4.3 Power [kWh/kg O'l ] 720 AOX @ inlet [Ilg/l] 276 AOX @ outlet [Ilg/l ] 62 AOX removal [%] 8.5 pH (*: mean of 9 mdependent trIals data sets) The results for AOX in leachate B before and after treatment are from samples analysed during one of the nine trials undertaken on this effluent, and simply demonstrate that AOX is generally readily decomposed by ozone in order to achieve the discharge consent concentration of 500 [Ilgll ]. CONCLUSION Air Product's CHEMOXTM system proved to treat very successfully one of the most difficult to oxidise waste waters, i.e. landfill leachate. These results were obtained by using the Impinging Zone Reactor (IZR). The particular IZR used in the pilot trials was not initially designed for treating leachate, i.e. the IZR proved to be very flexible. The local Gennan regulations of maximum COD concentrations for waste water disposal were undercut. The HRT was up to three times shorter and the power consumption almost 50 [%] less than the venturi/bubble column combination of the RSAG plant. This results in significantly lower capital costs for reactors and pumps. Additional operation cost savings are demonstrated due to lower energy input and a lower 03/COD ratio. Due to smaller surface:volume ratios for a full scale IZR, 03/COD ratios and energy savings could be reduced even further. Table 5 shows the comparison between the existing leachate treatment plant in St. Augustin and a CHEMOXTM system, both operating at the same COD removal rate. While the capital costs of both systems are equivalent there are significant savings in the operating costs. Table 5. Comparison of sizing and costing between a venturi/bubble column system and the CHEMOXTM system

COD removal [kg/h] O'l concentration in feed gas [wt%] Retention time [h] Enenzv costs [DMlkWh] O'l/COD rg;g] Operating cost saving [DMiyear]

Venturi / Bubble Column System 4 10 10.3 0.3 3.2

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Impinging Zone Reactor (CHEMOXTM) 4 10 4 0.3 1.8 320000.-

REFERENCES Barratt, P. A. and Xiong, F. (1995). Ozone Advanced Oxidation for the Treatment of Hard COD and Colour - Practica Experiences. Proceedings of the Sixth Annual Conference on Industrial Wastewater Treatment, Manchester, UK. !B( Technical Services, London. Clesceri. L. S., Greenberg, A. E. and Trussell, R. R. (1989). Standard Methods for the Examination of Water and Wastewatel 17th edition. American Public Health Association, Washington, USA. 5-10-5-16. Vogelpohl, A. and Gaddis, E. S. (1993). Veifahren und Vorrichtung zur biologischen Reinigung von Abwasser, German Pater No. 38 38 846.