Performance of an oxidation ditch retrofitted with a membrane bioreactor
during the start-up
ABSTRACT
The objective of this study is to elucidate the full-scale characteristics of an oxidation ditch (OD) retrofitted with a membrane bioreactor (MBR).
Domestic wastewater entering an oxidation ditch at a flow rate of 86 m3/d was directed to a MBR retrofitted into the original secondary sedimentation tank. The MBR contained flat sheet membranes. The data collected for 2 months during the start-up of the system showed that pH was maintained at
7.2 and 6.7 in OD and MBR, respectively. Dissolved oxygen (DO) in MBR
remained stable at 7.8 mg/L, while fluctuated in OD. The mixed liquor suspended solids (MLSS) in the OD remained steady at a concentration about1000 mg/L, but it was gradually building up from 500 mg/L to 2400 mg/L in the MBR during this period.Measurements of carbohydrate and protein were made by extracting the extra cellular polymeric substances (EPS)with sodium hydroxide (NaOH) from the mixed liquor obtained from both OD and MBR. Carbohydrate was predominant in the EPS and the ratios betwe
en carbohydrate and protein converged to fixed values from the fourth week; in this case the ratio was 4.5 for OD and 5 for MBR. The variation in EPS contents showed similar trends in both OD and MBR. The integrated treatment facility removed ammonia, COD and BOD at 100、91.6 and 97.0%, respectively. However, efficiency of nitrate and phosphate removal has not been realized yet.
KEYWORDS:Oxidation ditch; Submerged flat sheet membrane; Extracelluler polymeric substance; Nutrient removal
INTRODUCTION
An oxidation ditch is a modified activated sludge treatment process that is comparatively similar to wastewater treatment in
a sequential batch reactor. But the oxidation ditch provides
aerobic and anaerobic treatment in the same circular basin. The first full scale plant of oxidation ditch installed in Voorschoten, Holland, in 1954. There have been more than 9200 municipal oxidation ditch installations in the United States .Many oxidation ditches were operating around Australia during 1788–1988 and especially, more than 50 of these plants were in New South Wale .
A typical process flow diagram of an oxidation ditch is shown
in Fig. 1. Influent passing through a bar screen flows straight into an oxidation ditch (see Fig. 1a). Oxygen is added to the mixed liquor in the oxidation ditch using brush aerator which increases the surface area of the wastewater and creates waves and movement within the ditch . The aeration in oxidation ditch sharply increases the dissolved oxygen (DO) concentration but the DO decreases as the biomass uptake the oxygen when the mixed liquor travels through the ditch . After biodegradable organics or BOD is removed, mixed liquor will flow out of the oxidation ditch and sludge will usually be settled and removed in the secondary clarifier. Tertiary filtration after clarification may be necessary, depending on the effluent requirements.
The specific advantages of oxidation ditches include the following: long hydraulic retention time and complete mixing minimize the impact of a shock load, produces less sludge than the other biological treatment processes, energy efficient operations result in reduced energy costs compared to other biological treatment processes [4]. On the other hand, oxidation ditch has some disadvantages such as high effluent suspended
solids concentrations and requires large land area [4].
Nu0ria Vidal and co-workers (2002) compared the difference among three wastewater treatment processes including; (1) activated sludge process, (2) Ludzack–Ettinger (anoxic process followed by aeration process) and (3) Oxidation ditch [5]. For each of the three scenarios, the emissions and the use of natural resources due to their operation and transportation of sludge were evaluated. Consumption values for all energy resources were higher for the Ludzack–Ettinger configuration, while the values from the oxidation ditch configuration did not differ significantly from the activated sludge. Major environmental
impact by eutrophication due to NH
reactor 翻译4+/NO
3
-was reduced by 68% in the
Ludzack–Ettinger configuration and 75% in the oxidation ditch configuration with respect to the reference activated sludge process [5].
However, the quality of effluent from oxidation ditch is not sufficient enough for higher standards that ar
e required for unlimited irrigation and reuse of the water. Membrane bioreactor (MBR) technology has been applied to provide high quality effluent by constructing a membrane system into the existing clarifier.The benefits of using the MBR technology for retrofiting are no extra land requirement, produces high quality effluent for reuse, no odor, and allows treatment of much higher capacity of wastewater [6]. A full scale oxidation ditch retrofitted with a membrane bioreactor is in operation in Kilkenny Ireland since 1999 to treat 9000 m3/d of dairy farm wastewater [6].
The purpose of this study is to characterize the performance of a full-scale OD retrofitted with a MBR during the start-up. The study will show the attributes of treatment from raw wastewater to final effluent namely; COD, BOD, ammonia, nitrate, phosphate, EPS, soluble protein, carbohydrate, MLSS
concentration, DO , pH, turbidity, and permeate flux. METHODOLOGY
Experiments were conducted in the existing full scale OD retrofitted with    a submerged flat sheet MBR in the clarifier .Domestic wastewater passing through a primary screen (3 mm) entered the OD at a flow rate of 86 m3/d. Partially treated wastewater from the OD flowed to the membrane bioreactor. Membrane modules were operated with the available water head in the clarifier. The coarse bubble aeration was supplied to the membrane tank in order to prevent membrane clogging as well as to provi
de oxygen for microbial growth. Samples were collectedonce a week at the following 4 different positions: inlet and exit point of the OD, within the MBR and permeate outlet.
The MLSS concentrations were determined by filtering a known volume of a sample through a pre-weighed and dried 0.2-mm pore membrane (Whatman, Maidston, UK). These filters were then dried at 105℃ for 24 hours, and the increase in weight was measured [7]. Turbidity was measured using a turbidity meter (model 19977 micro TPI from scientific company). DO and pH were measured using DO electrode (model ED1 (Aqua-D) version 1) and a pH meter (modelIJ44 (Aqua-pH)) respectively, both from TPS company, Australia. Ammonia, nitrate and phosphate were determined by Palintest photometer (model 7000se, from Palintest company, England). EPS analysis by sodium hydroxide extraction followed Nagaoka’s method [8]. Modified Hartree-Lowry and Anthrone assays were applied for assessment of soluble protein and carbohydrate respectively [9,10]. Determination of Chemical oxygen demand (COD) and Biological oxygen demand (BOD) followed the standard methods for the examination of water and wastewater[11].
RESULTS AND DISCUSSION
Permeate flux
From Fig. 2, the MBR system shows a trend of fouling within
a month. Permeate flux dropped about 65.32 and 78.26% for each
operational cycle in the first and second month respectively. The change in MLSS concentration shown in Fig. 3 reveals that a relatively high MLSS concentration (such as on day 14 and 56) affects the membrane permeability dramatically. The results imply that the permeate flux variation is an inverse function of MLSS concentration. Similar result was found by In-Soung Chang and Su-Na Kim [12], who proposed that the MLSS concentration caused a cake filtration resistance on membrane and decrease in MLSS concentration led to a reduction in the resistance of filtration [12].
MLSS CONCENTRATION AND DO
As can be shown, the MLSS concentrations (Fig. 3) are interrelated to the DO concentration (Fig. 4) and pH (Fig.
5).Coarse bubble aeration was provided in the MBR system in order
to scrub and prevent biofouling on membrane surface. Due to greater DO levels in the MBR, the average MLSS concentration in MBR tank was higher than in OD. On the other hand, sharp increase i
n MLSS concentration can influence sudden drop in DO levels. For example, on days 14 and 56, the MLSS in OD and MBR reduced a lot of DO in the systems.
TURBIDITY
From Fig. 6, the inlet wastewater has lower turbidity than that of in the MBR and OD. Turbidity and MLSS concentration display similar trends, due to the fact that both are related with

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