Aeration costs

The impact of biomass concentration on energy costs is substantial and clearly an optimum biomass concentration exists that maximizes volumetric reaction rate while minimizing the aeration costs. Most full-scale MBR plants aim to operate at a biomass concentration of between 10 and 15 g [38]. The average a value at typical MBR for municipal wastewater with an MLSS concentration of 12 kg m MLSS for municipal MBRs is about 0.6 [39]. A design value for a of 0.5 has been recommended [40]. 

The practice of incremental feeding has not found favour in the production of brewers yeast as it has with bakers yeast. In bakers , in contrast to brewers yeast propagation, the growth medium is relatively inexpensive, the spent medium has little value and attemperation and aeration costs are high. Bakers yeast propagators tend to be large and fully utilized, unlike the small, intermittently used ones found in brewing. 

Jet Aerators. Jet aerators are a cross between the diffused and mechanical aerators. Air and water are pumped separately under the water surface into a mixing chamber and ejected as a jet at the bottom of the tank or pond (Fig. 3f). Jet aerators are suited for deep tanks and have only moderate cost. Disadvantages include high operational costs, limitations caused by tank geometries, and nozzles that can clog. Additionally, they require blowers. 

The closed-belt conveyor is not readily adaptable to the handling of sticky materials, and special designs may be required for materials which are highly susceptible to aeration. Initial cost per foot is relatively high because of belting cost, but power requirements are low and with proper instaUation and maintenance belt life is good. 

Bioprocess Control An industrial fermenter is a fairly sophisticated device with control of temperature, aeration rate, and perhaps pH, concentration of dissolved oxygen, or some nutrient concentration. There has been a strong trend to automated data collection and analysis. Analog control is stiU very common, but when a computer is available for on-line data collec tion, it makes sense to use it for control as well. More elaborate measurements are performed with research bioreactors, but each new electrode or assay adds more work, additional costs, and potential headaches. Most of the functional relationships in biotechnology are nonlinear, but this may not hinder control when bioprocess operate over a narrow range of conditions. Furthermore, process control is far advanced beyond the days when the main tools for designing control systems were intended for linear systems. 

A new concept is to use an on/oFf air supply cycle. During aeration, nitrates are produced. When the air is shut off, nitrates are reduced to nitrogen gas. This prevents acid buildup and removes nitrogen from the sludge. High power cost for aerobic digestion restricts the applica-bihty of this process. 

We have an aeration basin that currently operates at 3.2 mg/Liter DO. Compare this operation where the DO concentration is 1.3 mg./Liter. The temperature of the basin is 18.0° C and 200 kW of aeration power is used. The average electricity cost is 8 cents per kWhr. Determine (a) the current average electricity consumption for aeration (b) the daily electricity costs for the operation (c) what you could save on a daily basis and per year by lowering the DO concentration (d) determine the yearly savings on a percentage basis. 

Evaporation. The process of evaporation or distillation in the past was carried out in submerged-tube evaporators. These have been superseded by flash-type evaporators, which are more economical to run and reduce scale problems. The prcKess is suitable for brackish water, where the cost of chemical methods is excessive. The resulting distilled water is not palatable and re quires aeration to make it potable. 

Large power stations use complex feed heating systems before the boiler feed pumps (LP) and after the boiler feed pumps (HP), which can give high overall thermal efficiencies of 39 per cent. However, for the smaller machine, it becomes uneconomic to consider multiple bleeds from the turbine, and the final choice is dictated by the extra cost for the additional complexity against lower running costs due to increased efficiency. As a minimum, a contact type de-aerator is often employed which would extract a small bleed of around 2-3bar from the turbine. 

Oxygen can also be removed from feedwater by thermal de-aeration, or partially removed by skilful design of the feed heating system and blowdown recovery. These processes run without cost to the operator, but save chemicals, and, by reducing the required dose of sulfite into the system, decrease the amount of non-volatile solids added into the boiler. 

Although from theoretical considerations biomass yields from methane could be as high as 1.4, in laboratory-scale cultures values of about 1.0 were obtained, and in larger scale systems values were around 0.3-0.6. Methane fermentation also incurs high aeration and cooling costs. 

The most significant running cost of the fermentation is aeration, ranging from 71 to 92% of fermentation running costs. See Table 4.12. 

Aeration comprises 88.9% of the running costs of fermentation (Table 4.12). Running costs of fermentation comprise 67.9% of the Total running costs (Table 4.11). Running costs comprise 23.8% of the total production cost (Table 4.9). 

Aeration contributes most to the production cost of the yeast, at 0,060 per kg. Cooling comprises least, at 0,005 per kg. 

The tray aeration method is a simple, low-maintenance method of aeration that does not use forced air.19 Water is allowed to cascade through several layers of slat trays to increase the exposed surface area for contact with air (Figure 18.9). Tray aeration is capable of removing 10 to 90% of some VOCs, with a usual efficiency of between 40 and 60%.53 This method cannot be used where low effluent concentrations are required, but could be a cost-effective method for reducing a certain amount of VOC concentration prior to activated carbon treatment. 

---