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The treatment of wastewater worldwide and in the Caribbean typically consists of aerobic methods for BOD removal followed by an anaerobic treatment for the sludge produced. Lower energy requirements, lower production of sludge, along with the production of methane are advantages of anaerobic systems. Despite these advantages however, anaerobic systems have not traditionally been the customer's choice, as they demand higher capital costs. With the recent advent of high rate anaerobic systems, the capital cost has been drastically reduced. These high rate reactors include the Anaerobic Contact Tank, Anaerobic Filter, Expanded or Fluidized Bed Reactor and the Upflow Anaerobic Sludge Blanket Reactor (UASB).

The influent enters the UASB at the bottom and passes through a sludge blanket containing a high biomass concentration. Due to the anaerobic reaction, methane is produced. As the biogas and effluent rises in the reactor some of the biomass also rises. The special feature of the UASB is the gas, liquid, solid separator which enables retention of the biomass within the reactor giving way to low hydraulic retention times. The main disadvantage of the UASB is that the escape of fine organic colloids contributes a high percentage of the effluent BOD. In order to meet the effluent quality standards, post treatment in the form of oxidation ponds is hence required.

The objective of this paper was to design, fabricate and test a reactor with a configuration that avoids separate post treatment.

Before seeding the reactor, a qualitative tracer study was undertaken where a concentrated dye was injected into the inlet stream. The flow of dye through the reactor showed visually that there were no areas of stagnation in the reactor. However, a quantitative tracer study using NaCl was also undertaken to determine the flow regime in the UASB reactors.

Anaerobic seed sludge was prepared in the Environmental Engineering Laboratory of UWI by collecting sludge from the anaerobic digester at the Arima domestic wastewater treatment plant. This was transferred to six 2L bottles and fed with 300-600 mg/L glucose as a carbon source. Sodium carbonate was used as a buffer to keep the pH in the range 6.5 – 7.5. The sludge was maintained on glucose feed till it was used to seed the UASB.

After hydraulic testing was completed, sludge was transferred from the batch reactors to the UASB. The synthetic wastewater consisted of a glucose concentration of 300 mg/L for the first two weeks, after which this was increased to 500 mg/L. This was supplemented by an inorganic nutrient medium.

The UASB was operated over a 33 day period at a flow rate of 37 L/d, which gave a hydraulic retention time (HRT) of 14 hours. The total suspended solids (TSS) of the seed sludge to the UASB reactor was 34.5 g/L and the Volatile Suspended Solids (VSS) was 18 g/L. Using the flow rate, influent COD and VSS, the space loading rate was calculated to be 3.10 kg COD /m3d and the sludge loading rate was calculated to be 0.19 kg COD/kg VSS d. Gas Production was found to be around 10 L/d at its highest, and 2 L/d on average. COD, pH, alkalinity and turbidity were monitored every day.

The results indicate that the UASB has the ability remove both soluble and total COD. Also, it can be seen that after day 20, the soluble COD and total COD for the reactors are closer. This is an indication that the majority of the lighter fractions have been flushed out of the reactor leaving behind the larger particles that will undergo granulation. The average percentage soluble and total COD removed were 55% and 40% respectively. According to van Haandel and Lettinga (1994) a COD removal efficiency of 35% was obtained after 33 days. Grasius (1996) obtained efficiencies of around 60 % for the as time frame. This shows that, for the same time frame the reactor behavior is in keeping with anticipated values.
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Document Type: Research Article

Publication date: 2001-01-01

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