Overview of BioTower Project

In order to exceed our requirement to the EA through the IPC authorisation of maintaining COD below 30,000 mg/l, and reduce waste effluent charges to a commercially acceptable level, we decided to begin planning and development of a Biotower. Often also referred to as a biofiltration system. These biological treatment processes can be used for many purposes, in this case the desired effect was to reduce COD. The process involves biological film upon a plastic media that reduces the biological oxygen demand (BOD) of the effluent, which in turn reduces the COD as both are related closely to each other. This is the reason that not all COD can be treated, because it is only possible to treat the COD which is related to BOD, an average calculation of the ratio of COD to BOD in our effluent is 1.8-1. The only bi-product of the process is sludge that will be treated in the sludge de-watering unit.

To see a copy of the performance data for 2002 click here

The Biotower in Practice, How it Works, and Parameters

The key to successful management of a Biotower or similar system, is to control the parameters in such a way to create the perfect, or as near perfect as possible, environment for maximum effectiveness, in this case maximum COD removal, and prevention of bacteria death.

Key parameters include moisture, pH, COD (supplemented by nutrients), trace metals, and temperature. Moisture is maintained by the constant trickling of effluent down through the media. If for any reason the waste water stops circulating, the media should be ideally be wetted for 10 minutes per 24 hrs.

Picture: The Rotary distributor, with effluent being trickled over the bacteria covered plastic media.

pH is probably the most important factor to control. An ideal ph would be 7, however sulphate-reducing bacteria which can cause odour problems thrive at this pH and can produce strong odours, but keeping pH above 7.8 eliminates these bugs and should therefore keeps odour to a minimum. Any pH above 8.5 however would be too alkaline and would begin to kill off biomass. To avoid these potential problems, operating pH levels have been set between 7.8 and 8.5. pH is controlled by adding either hydrochloric acid (to lower) or caustic soda (to raise) pH. The absolute extremes of pH are 7 - 9.5, below pH 7, fungal growth (less efficient at removing COD) will appear, and above pH 9.5, bacteria will be killed.

COD is the food supply, There is a risk of starvation or overfeed, depending on applied COD loading, the Biotower should cope with a doubling of the loading over 24hrs, but there will also be a noticeable increase in the sludge yield. An indicator of overfeeding, is high levels of ammonia in the final discharge effluent. The required nutrients are soluble forms of nitrogen and phosphate, the recommended forms being ammonium nitrate and diammonium phosphate, sourced from agricultural suppliers for best value. If too much nutrient has been added then high levels of ammonia will be present in humus tank. The biological matter also needs trace metals such as zinc, potassium, and iron. These are found in trade effluent, often in abundance, and hence no additives of this kind are required. The nature of wastewater entering the tower is such that no significant enough change in temperature should occur. Any temperature changes that do occur will be gradual and small. Temperature is maintained by both the heat that the bio matter generates, and the temperature of the wastewater itself. Optimum operating temp for aerobic bacteria is 15-20C, historical effluents range from 8-30C, which at the extremes should at worst reduce efficiency. A temperature any lower than 4°C however would significantly reduce efficiency.

Documentation, Ensuring Sound Usage and Effectiveness

Documentation is a key component of any project in industry due to the level of regulation of both the environment and of health and safety. For this reason, and the complexity of the Biotower, a detailed operating manual, running sheet, and maintenance guidelines had to be prepared to ensure that the Biotower was run correctly. A document entitled ‘General understanding of Biotower’ was also produced to help shift workers with little experience of how the Biotower works, to understand the process and supplement the operating manual which is designed for the operator who will already have an understanding of the basic processes.

Benefits and Success of Project

The project has so far been a significant success, lowering COD levels from the often 25,000 - 30,000mg/l, which were previously the norm, to levels from 8000 - 14,000 mg/l. This well exceeds the 30,000mg/l limit set out by the IPC authorisation. Cost savings have also been significant with monthly effluent charges now being projected at under approximately £20,000, still well above the £2000 per month prior to the use of the Mogden formula, yet still significantly lower than the £35,000 had the Biotower project not occurred at all. Minor problems have occurred, such as damage to pumps, and electricity supply limiting efficiency and testing failures, however, once these teething problems have been overcome, still further COD reductions and better compliance should be achieved.

Pete Jones - November 2001