System Design Approach Using Ultraviolet and Ozone for Waste Water Reuse Application

Dr. Achim Ried*, Taeyoung Choi**

* ITT-WEDECO GmbH, Boschstr. 6, 32051 Herford, Germany, achim.ried@itt.com

** ITT-WEDECO AG Shanghai office, 31F, Building A, Hongqiao City Center of Shanghai, Zunyi Road No. 100, Shanghai, China, taeyoung.choi@itt.com

ABSTRACT

The use of ozone and/or UV process for waste water reuse application is often a combination of ozone and/or UV-step with additional treatment steps, e.g. biological treatment, flocculation, filtration and activated carbon. Therefore, it is necessary to develop an optimized combination of these different steps. This article is going to demonstrate an overall process design approach for waste water reuse application presenting two examples for municipal waste water treatment.

Keywords: ozone, UV, Ultraviolet, waste water reuse, municipal waste water, process design approach

INTRODUCTION

Municipal waste water is often a complex mixture of waste from households and different kind of industries. Conventional treatment techniques include coagulation/flocculation/sedimentation processes, biological steps and filtration steps. Due to the improvement of waste water treatment techniques and the extension of regulations given by the authorities, nowadays further treatment steps are under discussion, e.g. membrane filtration, UV-disinfection, ozone oxidation, adsorption.

A well-known example of such regulation is the EU bathing waters directive. For effluents from waste water treatment plants to be discharged into bathing water, the standards applicable to germ reduction must be met in the treated water. Also, for the reuse of municipal waster water, e.g. for agriculture, strict limits have to be met (Liberty et al. 1999).

Another topic that throws light on a debate of the need of further treatment techniques is connected with substances that are not or hardly biodegradable. They are referred to as persistent substances. These may be of various anthropogenic origins. The main compounds/areas of origin discussed in this connection include industrial chemicals (e.g. nonylphenols, organotin compounds, PCB, phthalates), pesticides (e.g. DDT, Gammexane), medicines and cosmetics.

Furthermore public debate is currently focusing on what are called endocrine disrupters (EDC). The term “endocrine” denotes substances that affect the hormone system (Schlumpf and Lichtensteiner, 1996). Negative effects on the hormone system of fish in surface water have been reported (Seibert, 1996). Effects such as reduced fertility of fish are to be observed in particular in the area of influence of sewage plant outlets. Possible effects on humans are still the subject of controversy.

In the case of endocrine substances in particular, no limits currently apply. Nor are there any guidelines on possible treatment techniques.

Principle Aspects of Ozone and UV Barriers in Municipal Waste Water Treatment

In principle recycling of polluted water is mostly limited or even excluded by contamination of persistent substances, germs, odors and/or color. This limitation is evident as long as no additional treatment is available. Table 1. shows some principle treatment steps and the main effects of ozone and UV.

Table 1. Treatment steps for municipal waste water and principle effects

Primary treatment

Biological treatment

Clarifier Filter

Additional treatment

Technique

Flocculation Coagulation Sedimentation Filtration

Activated sludge

Fixed-bed Different kind of reactors

Sedimentation

Sand filter

Different membrane filters

Membrane Chlorination

Ozone

UV

Adsorption (AC)

Effects Ozone

COD removal

Removal of specific persistent compounds Disinfection

Further color/odor removal

Improvement of UVT(%)

Effects UV

Disinfection (Photolysis, Photo­oxidation)

Status of Research and Pilot Studies

During the past years different projects were conducted to investigate the usage of ozone and/or UV process(es) to treat waste water effluent additionally for reuse application.

Ozone is proven to be very effective in oxidation of various compounds. The oxidation process leads to oxidized molecules, which loose their estrogenic and toxic effects. In addition, they are mostly better biodegradable (Bila et al. 2004; Jasmin et al. 2005).

One main pathway for pharmaceuticals and endocrines are effluents of municipal waste water treatment plants (POSEIDON 2004). These so-called secondary, biologically processed effluents end up typically with a reduction by 70 to 90 %, corresponding to the concentration of unwanted substances. But, still the residual concentrations in the range of µg/L or ng/L can have a negative effect on the environment, e.g. fish populations.

Possible additional treatment options are the usage of membranes (nano-filtration or reversed osmosis), flocculation/filtration, adsorption on activated carbon, and oxidation. The oxidation process has the advantage to be an in-situ treatment solution without generating contaminated concentrates or polluted adsorber materials. The oxidation process is favorable if the oxidant reacts specifically with the contaminant, the required amount of oxidant to achieve the treatment goal is economical, and formed by-products are none toxic for the environment (Ternes et al. 2003; POSEIDON 2004; Bahr et al. 2005; Zhang et al. 2005).

The ozone treatment step can be installed after a clarifier. Von Gunten (von Gunten et al. 2005) showed that smaller amounts of SS (up to 60 mg/L) have no significant negative influence on the treatment results.

In case the process chain of the waste water treatment plant consists of biology, clarifier and an additional sand filter, it is sufficient to implement the ozone step between the clarifier and the sand filter. It is proved that after the ozone treatment a certain part of the residual DOC (up to 60%) is biodegradable in soil columns (Schumacher, 2003). A treatment step subsequently to ozonation, which enables the reduction of the formed biodegradable DOC, would improve the treatment results furthermore. For stricter limitations on disinfection, UV process could be adapted after the bio-filter. Since the ozonation will improve the UV-transmittance the final UV reactor can be operated with less lamps and energy. Table 2. shows typical dosage range of UV and ozone for additional treatment of municipal waste water effluent.

Table 2. Treatment steps for municipal waste water and principle effects

Treatment Goal

Available Technique

Dosage Range

Disinfection

UV

(low pressure)

300 – 400 J/m²

COD reduction

ozone

50 – 150 g/m³

Colour reduction

ozone

10 – 50 g/m³

Reduction of persistent

substances

e.g. endocrine, antibiotics

ozone

10 – 15 g/m³

Reduction of parasites

e.g. threadworm

ozone

10 – 15 g/m³

The following two practical examples will give a more detailed picture of advantages using ozone in the municipal waste water treatment.

Case Study I : Tubli plant / Bahrain

The Ministry of Works and Agriculture of the State Bahrain decided to use an ozone treatment step for the municipal waste water treatment plant Tubli. The total treated water flow is 200.000 m³/d

(300.000 m³/d future extension). The traditional primary and secondary treatment steps cannot reach the limits given by the World Health Organisation Guidelines in association with California standards and United States Environmental Protection Agency Guidelines for the unrestricted agricultural irrigation.

Beside certain types of bacteria, the main focus, in this case, is the removal of the helminth “Strongyloides stercoralis”. The established treatment techniques are not able to remove it to acceptable limits (< 1 helmith/L).

Pilot tests conducted by Associated Consulting Engineers (ACE) showed that in comparison with chlorine ozone is the more powerful agent for helminth removal (R. J. Abumaizar et al, 2003).

The experimental results illustrate strongyloides stercoralis removal rates of more than 95% with an ozone dosage of 10 mg/l and 25-30 min reaction time. For the same removal rate, a chlorine dosage of 30 mg/l and a reaction time of 120 min were determined.

For that reason an ozone plant with an overall capacity of 144 kg ozone/h (3 times 48 kg/h) was installed. The designed ozone dosage is 12 mg ozone per litre secondary effluent. For gas introduction, a diffuser system is used. The overall ozone dosage of 12 mg/L is split into two dosage points – into a pre-ozonation and a post-ozonation step (see Fig. 1). Each ozonation step comprises a 3-chambered contact basin. In each case the retention time is around 25 minutes.

Fig. 1: Flow chart of ozone plant in Bahrain

Fig. 2: Ozone plant in Bahrain (WEDECO Effizon HP generators)

Case Study II : Kalundborg / Denmark

The water treatment plant Kalundborg purifies and clarifies mixed sewage consisting of 20% municipal and 80% industrial waste water. The industrial fraction largely derives from a sizable international pharmaceutical company, which operates one of the largest insulin production plants worldwide at this site.

Although the character of Kalundborg is that of a smallish town, the treatment plant is designed to purify and clarify the waste water of nearly 350.000 residents resulting from the adjacent pharmaceutical company. The waste water contains difficult-to-degrade organic impurities monitored as COD value. Due to the extension of production scheduled by the pharmaceutical company, the authorities of Kalundborg had decided to redevelop the treatment plant completely. As last treatment stage an ozone treatment was projected within the recirculation system, for degrading a volume of up to 1,250 kg COD per day. Therefore the ozone system was designed for 180 kg ozone/h (2 times 90 kg/h).

After passing through treatment stages typical for water treatment plants, the biologically pre-clarified waste water is treated by ozone within the recirculation system. This ozonation takes place in six reaction vessels with a total volume of 300 m³. Within a period of only 15 minutes of total contact, a drastic reduction of the stubborn organic impurities occurs. The COD level before the oxidation step is between 100-150 mg/l. The treatment goal is to achieve a COD < 70 mg/l.

Such types of waste water not only contain organic impurities as mentioned above, but can also contain drugs and hormonally active residues, so called “endocrine disruptors”. By conventional purification and clarification processes these impurities are not degraded sufficiently and endanger the environment. Research results prove that ozone is an appropriate means to degrade these harmful substances in municipal waste water. Thus the oxidative treatment of waste water by ozone plays a decisive role in multi-barrier concepts (A. Ried, T. Ternes et. al, 2003)

Fig. 3: Flow chart Kalundborg

CONCLUSIONS

As advanced treatment of municipal waste waters, the use of ozone and UV enables waste water effluent quality to meet exiting and future recycling standards. Especially, the existing regulations for disinfection and the possible future regulations for different kind of persistent substances , e.g. industrial chemicals, hormones and pharmaceuticals, can be achieved by ozone and UV treatment.

Beside the principle, well-proven treatment effects, cost calculations and operation experience have to be considered in designing additional processes for waste water reuse application demonstrate the overall advantages of ozone and UV treatment.

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ACKNOWLEDGEMENT

The authors are grateful to the Associated Consulting Engineers/Bahrain and the municipality of Kalundborg/Denmark for their contribution, for providing data and for good co-operation.