A man-made natural fibre producer had produced a new high strength, fibrillated cellulosic fibre derived from wood pulp destined for use in garment manufacture. As a result of the IPC authorisation attached to the production of this product it became necessary to establish the biodegradability of various effluent streams leaving the production plant. In order to evaluate the feasibility of treating effluents arising from the plant aerobically it was decided to install a pilot scale Immobilised Cell Bioreactor (ICB).
Cleveland Biotech were contracted to provide appropriate bacteria to seed and dose the reactor and also to provide ongoing technical advice throughout the duration of the project. The object of the trial initially was to obtain a high level of COD removal but it was later defined as the removal of at least 75% of the BOD.
The ICB pilot unit had a holding capacity of 30,000 litres split into 3 aerated compartments of 15,000, 7,500 and 7,500 litres, the largest section being the inlet chamber. Each chamber was filled with carbon coated foam/spacer mixed media. The effluent flow was bottom up concurrent with the airflow. Neutralised effluent (NE) from the plant was fed from buffer storage tanks into the first compartment of the ICB after pH adjustment. The effluent contained a complex mixture of cellulose breakdown products including lignin, amines and amides.
After initial test work and laboratory trials, work was sufficiently advanced by August to begin aerobic treatment trials of the effluent using the ICB pilot unit. Cleveland Biotech provided specialised bacteria, Amnite C350, for the initial seeding of the IBC and for continued dosing throughout the trial. In order to allow the establishment of a biologically active biofilm within the packing matrix it was decided to introduce the COD load to the reactor in a gradual fashion.
The aeration rate was set at 80scfm and, in view of the level of organic impurities in the NE liquor, it was decided that a macro-nutrient supplement to the pilot unit was necessary. This supplement, once continuous production was adopted, was eventually fixed at a rate of 250ml/m³ effluent treated
It was also agreed to monitor closely the temperature levels in the pilot unit as temperature in the treatment plant could fall as low as 8.9°C in cold weather whereas optimum temperature for biodegradation is 25°C. Over the first 2 months of the trial a gradually increasing proportion of neat NE liquor was fed batch-wise to the pilot plant. Measurements of the TOD/COD of neat NE liquor was collected and combined with measured TOD/COD from the aerobic reactor to give a measure of COD destructivity.
Initial degradation rates were low. With tests showing that no obvious inhibitors were present, other than lower than optimal temperatures, it was concluded that the bacteria were taking time to adapt to the high levels of recalcitrant organics (hard COD) present in the waste stream. This proved to be the case viz:
Days 32 > 52: from starting COD of 1321ppm to 610ppm – 52% removal over 12 days
Days 53 > 57: from starting COD of 2161ppm to 1321ppm – 60% removal over 4 days
The results showed a significant increase in performance of the system over time with respect to TOD/COD removal but they needed to be enhanced further.
As biological systems generally operate most efficiently under constant or steady state growth conditions it was decided to move to a continuous forward feed of 0.20m³ NE liquor/h giving a 5 day residence time and to advance the forward feed rate progressively to establish optimum destructivity/hydraulic residence time (HRT) relationship.
The most effective HRT was found to be 2.5 days and it was agreed to continue trials feeding NE liquor forward at this rate. It was also agreed to put in heating into the ICB to maintain the temperature at an optimal 25°C in order to maximise the degradation rate. Emphasis at this stage switched from COD reduction to BOD reduction.
With the BOD removal efficiency across the ICB pilot plant at a respectable 89% the trial showed that the aerobic treatment process, when bioaugmented with appropriate degrading bacteria, would produce positive results if adopted for treatment of the effluent streams from the main treatment plant
Though the trials were not processed further it is possible that the reduction in the COD levels could be increased well above the 58% to 60% achieved in the trials by use of an enhanced microbial product. As the measurement of BOD in the BOD test is obtained by using a ‘standard’ bacteria culture the result represents the ability of the ‘standard’ bacteria to break down the organics in the sample. These ‘standard’ cultures are not specifically formulated for certain ‘hard’ components in the effluent and therefore may register a lower BOD than if specifically formulated bacteria are used.