AERATION OF AEROBIC BIOLOGICAL WASTEWATER TREATMENT PLANT

INTRODUCTION

(PICTURE TAKEN FROM WWW.WASTEWATER.COM)

Aerobic biological wastewater treatment is one of the most popular options in wastewater treatment. Biological treatment usually constitutes the secondary treatment stage in a conventional wastewater treatment plant. However, despite its widespread use and popularity of choice, the aerobic biological wastewater treatment plant is often befuddled by many problems leading to inefficient treatment process.

One of the main causes of failure in aerobic biological wastewater treatment plant is due to the poor understanding of the basic processes involved. This often results in the poor design and operation of the aerobic biological wastewater treatment plant.

In this article, we will discuss the various problems associated with the aeration processes in aerobic biological wastewater treatment plant.

Aeration process of the wastewater treatment plant should not be looked simply as just pumping of air into the wastewater treatment plant. Poor understanding of the aeration requirements have often resulted in plant failures and economically inefficient operation.

It is a common knowledge that most plant operators seemed satisfied as long as they see large bubbles of air generated in their treatment plants. Treatment plant operators still could not understand why their treatment plants failed despite being continuously aerated. Most plant operators do not even know that their treatment plants are in fact not fully operating under aerobic conditions.

Poor aeration management of treatment plant is often cited as one of the main causes of treatment plant failure. Problems generated by aeration failure in biological wastewater treatment plants range from emission of foul odours to failure to comply to effluent discharge standards.

There are no serious attempts to optimize or monitor the aeration processes, unless the aeration system is out of order. Aeration of wastewater treatment plant has always been considered as one of the costliest components in treatment plant operations and yet very little attempts are made to optimize the process and make it cost efficient.

This scenario occurs because of the lack of understanding and the impact of the aeration processes on biological wastewater treatment.

AEROBIC BIOLOGICAL WASTEWATER REACTORS

For the waste treatment to be effective, the reactions will have to be carried out by high concentration of microorganisms. This is only possible if the microorganisms are cultivated in a wastewater bioreactor which is designed to support the growth of high concentration of microorganisms.

The wastewater bioreactor is able to support high concentration of microorganisms and treat high concentration of wastewaters within small space confinement because:

1 Provision of good mixing and circulation to ensure that nutrients

and oxygen will reach every microbe in the bioreactor

2 Provision for intense aeration to supply all the oxygen needs of all

the microorganisms

There are various types of wastewater biological reactors that exploit the activity of the aerobic microorganisms in treating wastewaters. The wastewater bioreactors are either based on attached or suspended microbial growth forms such as found in biofiltration and activated sludge systems respectively.

In our discussions we will be concentrating on the Continuous Stirred Tank Reactor as exemplified by the activated sludge system

PRINCIPLE OF AEROBIC BIOLOGICAL TREATMENT

The crucial element in all these aerobic biological wastewater treatment systems depends on the ability of the system to provide sufficient oxygen to support the growth of the aerobic microorganisms. Although oxygen is supplied in the gaseous form to the wastewater, the microorganisms are only able to metabolize the oxygen in the form of dissolved oxygen.

Generally it is agreed that a minimum steady state dissolved oxygen value of about 2 mg/litre should be maintained throughout the mixed liquour to support the growth of aerobic microorganisms. If the value of the dissolved oxygen falls below 2 mg/litre, the wastewater treatment system might be anaerobic and will instead support the growth of anaerobic microorganisms.

Even if the dissolved oxygen value is maintained above 2 mg/litre, throughout the mixed liquor, there are anaerobic zones developing within the microbial aggregates due to poor mass transfer of dissolved oxygen from the surrounding environment into the microbial aggregates.

PROBLEMS OF OXYGEN SUPPLY

In reality, it is difficult for wastewater bioreactors to achieve uniform dissolved oxygen concentration throughout the bioreactor due to the large size of the bioreactor, poor mixings and liquid circulations within the bioreactor. This will often result in oxygen gradients and stratifications being formed within the bioreactor.

Another main contributory factor to the low dissolved oxygen in the mixed liquour is attributed to the poor solubility of oxygen in water. Oxygen is a gas under normal temperature and pressure. Even with very clean water, the saturation value of dissolved oxygen is only about 8 mg/litre. In dirty wastewaters the amount of dissolved oxygen at saturation is far less due to the presence of high concentration of dissolved solids.

The presence of oily films on the surfaces of wastewaters also helps in reducing the mass transfer of oxygen into the mixed liquor.

Dissolved oxygen readings above 2 mg/litre are often exhibited at the surfaces of the mixed liquour and within proximity of aeration or mixing zones. These liquid-air interphase zones exhibit higher oxygen diffusion due to efficient mass transfers. At deeper zones the dissolved oxygen readings could decline rapidly making the bulk of the bioreactor anaerobic.

Low dissolved oxygen values could also be attributed by higher oxygen utilization rate due to higher number of microorganisms or high substrate loadings. The rate depletion of dissolved oxygen is not matched with the rate of oxygen replenishments to the system

There is often not enough dissolved oxygen to sustain the growth of all aerobic microorganisms throughout the reactor despite the visual observations that the aerators are fully working.

PROBLEM OF SUPPLYING OXYGEN IN AEROBIC BIOLOGICAL WASTEWATER TREATMENT PLANTS

There is a huge reservoir of oxygen gas in the atmosphere. Oxygen constitutes about 20% of the composition of air. However, these huge supply of oxygen could not be efficiently exploited by the microorganisms due to:

1 Microorganisms can only use oxygen in the form of dissolved

oxygen in their metabolism

2 Oxygen under normal temperature and atmospheric pressure is a

gas with very limited solubility in water.

The concentration of dissolved oxygen in water is very low. Only very clean water will be able to maintain dissolved oxygen saturation value of about 8mg/litre. In dirty and polluted water the saturation dissolved oxygen value could be very low.

Oxygen from atmosphere can enter the wastewaters either by passive diffusion or by forced aeration. Forced aeration may be brought about by mechanical mixers or by compressed air into the wastewater.

In both these methods, the mass transfer of oxygen to the wastewaters is still by diffusion and governed by the laws of diffusion.

The oxygen diffusion process is based on the molecular motion of molecules with the direction of diffusion dictated by the direction of the concentration gradient.

The diffusion process is affected by various physical parameters such as:

1Thickness of diffusion barrier

2 Surface area of diffusion

3 Difference in concentration of diffusion molecules

4 Temperature

5 Time

OXYGEN PATH

We can follow the fate of oxygen from its source to the targeted microorganisms by following the diffusion path of the gas. Analyzing the diffusion path will help us determine the rate limiting step where improvements can be made.

The oxygen path for forced aeration as in the activated sludge system is as shown:

Air (Bubble)®Mixed liquour®Mixed liquour/floc interphase®Floc/microorganism interphase

For oxygen to be transferred from the air bubble to an individual microbe, several independent partial resistances must be overcome.

Resistances for oxygen transfer from air bubble to the microbial cell:

• Resistance within the gas film to the phase boundary

• Penetration of the phase boundary between gas bubble and liquid

• Transfer from the phase boundary to the liquid

• Movement within the wastewater medium

• Transfer to the surface of the cell

At each step of resistance encountered will result in the sharp decrease of oxygen diffusion rate, which will lead to the decline in oxygen transfer. This would mean lesser oxygen servicing the microorganisms.

In aerobic biological wastewater treatment, the most inefficient stage of the diffusion path is usually movements through the mixed liquour.

The distance covered and the viscosity of the wastewater could affect the efficiency of the oxygenation process resulting in little or no oxygen reaching the microorganisms.

MASS TRANSFER OF OXYGEN FROM AIR BUBBLES

The most common method of supplying oxygen to activated sludge systems is by the forced aeration. Air bubbles are generated as compressed air is released at the nozzles or spargers.

Initially, small air bubbles are generated. These small bubbles rapidly increase in size as it rises to the surface. On reaching the surfaces, these bubbles collapse, releasing the gas into the atmosphere.

Gas exchanges occur between the gas bubbles and the surrounding medium as the bubbles rise to the surface.

Oxygen being higher in concentration inside the gas bubbles will diffuse out of the bubbles into the surrounding environment where the concentration of oxygen is lower. The lower concentration of oxygen in the medium is attributed to the rapid consumption of oxygen by the microorganisms in the mixed liquour.

Carbon dioxide is generally higher in the mixed liquour compared to the concentration of carbon dioxide in the gas bubbles. The higher concentration of carbon dioxide is due to the product of microbial metabolism which generates carbon dioxide. This difference in concentration will result in the diffusion of carbon dioxide into the gas bubbles from the surroundings.

The efficiency of mass transfers through the gas bubbles is affected by various parameters. Smaller size bubbles have more efficient diffusion rates compared to large bubbles due to their high surface area to volume ratio.

At the point of bubble generation where the bubbles are smaller, the mass transfer of oxygen is highest as small bubbles exhibit higher surface area to volume ratio.

As the bubbles of air rises it expand in size resulting in a lower surface area to volume ratio resulting in less efficient mass transfer of oxygen

Fresh air bubbles surfaces are usually free from deposition of organic matter or foam. However, with time the bubbles surfaces will be rapidly coated with layers of foam. This will result in a thicker bubble wall which will impede further the gas exchanges occurring between the bubble and the environment.

Refreshment of bubbles could occur in the mixing zone where old bubbles collapse and new bubbles regenerated.

The efficiency of the oxygen diffusion process is affected by the amount of time the bubbles resides in the mixed liquour. Longer bubbles residence time would mean longer time for gas diffusion to occur

The bubbles residence time is affected by the length of the oxygen path or distance traveled by the bubble. Longer oxygen path would result in longer bubble residence time.

Smaller bubbles due to the higher surface area to volume ratio will be exposed to higher drag forces acting on the bubbles as it rises. This explains why small bubbles rise slowly. Bigger bubbles have lower surface area to volume ratio and less drag forces resulting in rapid rise of air bubbles and shorter bubble path and time for diffusion of oxygen to the surroundings.

The bubble path can be extended by intense mixings of the mixed liquour. Under intense mixing, the bubbles generated will be subjected to a long and tortuous path, thus improving the diffusion process.

IMPROVEMENT OF MASS TRANSFER BY MIXING

Mixing of the mixed liquour will also confer other advantages to the aeration efficiency of the wastewater treatment system

In a turbulent system, the mixed liquour is agitated by a complex combination of aeration, stirring and baffles systems. Transfer of oxygen into the broth is brought about by:

· Bubbles of air

· Entrainments of air

· Shear reactions

GASES ENTRAINMENT

During active agitation, the mixed liquour will show a chaotic dispersion of fluid. The surface of the mixed liquour will be destabilized and fluid vortex formed by stirring will be disrupted. In such situations gases such as oxygen will be easily entrained or trapped within the fluid motion resulting oxygen diffusing effectively into the mixed liquour.

SHEAR ACTIONS

High turbulence resulting in high shear forces will be generated during active agitation. Shear forces occur due to development of eddy currents, laminar shearing, bubbles dispersions and stirrer generated shearing. All these shearing will result in the fluid being stretched thin, frictional forces between the phases resulting in higher amount of gas diffusion.

LEVELS OF MASS TRANSFERS IN BIOREACTOR

During the mass transfer of oxygen through the mixed liquour to the microorganisms, there are two main stages of mass transfer. The first stage is governed by the homogenous macro mixing of the mixed liquour to ensure that oxygen and other nutrients reach every point in the bioreactor. The second stage of mass transfer is observed at a microscopic scale in the intimate proximity of the microorganisms. At this stage, mass transfer process of oxygen is affected by the boundary layer effect which influences the diffusion process of oxygen molecules to the microorganisms. Only shearing forces brought about by mixing will reduce the thickness of the boundary layer to improve the diffusion of oxygen into the microbial cells

WHY TREATMENT PLANT NEED TO BE CONTINUALLY AERATED

The very low solubility of oxygen in water is considered the greatest challenge in supplying oxygen to the microorganisms in aerobic biological wastewater treatment plant. This is often the costliest rate and rate limiting step in the operation of the treatment plant.

Due to these problems the treatment plant has no choice but to continue aerating despite very little of the oxygen pumped will be used by the microorganisms. A lot of oxygen is just wasted unused and returned to the atmosphere.

Oxygen utilization rate studies have shown that the microorganisms rapidly utilize oxygen in the wastewaters despite the low steady value of dissolved oxygen in the mixed liquour. It is therefore the problem of solubility of oxygen gas to form dissolved oxygen as the rate limiting step

The amount of oxygen which can diffuse into the wastewaters at steady state therefore depends on the amount removed or utilized by the microorganisms. Those unused are simply returned wasted to the atmosphere.

The alternative solution taken is to fine tune the supply and demand of oxygen to make the treatment process efficient and economical. This requires close monitoring and understanding of the process in order to control the amount of oxygen supplied as required.

FINE TUNING THE OXYGEN SUPPLY

Fine tuning of the oxygen supply is where when the amount of oxygen provided should be equivalent to amount needed by all the microbes. Nothing more and nothing less. In such situations excess oxygen is not wasted and there will be no lack of oxygen to jeopardize the treatment process.

This objective can be achieved if we know just how much oxygen can be utilized by the microorganisms in a unit volume of wastewaters and how much oxygen can be transferred to a unit volume of wastewaters at any one time during the treatment process.

The measure of how much oxygen needed is measured by the oxygen utilization rate or OUR, and the measure of the oxygen transfer into the wastewater by oxygen transfer rate or OTR. Ideally OTR should equal OUR.

The values of OUR and OTR could be affected by various parameters such as composition of wastewaters, microbial content and phase of growth among others. Thus field and laboratory studies have to be carried out for each specific wastewaters and operating conditions. Close on site monitoring of the process is essential to keep the treatment plant working within the optimal range of parameters.

IMPROVING THE AERATION PROCESS

There are a few strategies that can be taken to improve the aeration efficiencies of biological wastewater treatment plant

Improve sparger design that will result in the generation of smaller size bubbles. These small size bubbles will have higher surface area to volume ratio leading to better mass transfer of oxygen.

Improve stirrer’s design that will encourage good mixings and fluid circulations

Improve design of bioreactor that will promote good flow circulation

and avoiding dead ends.

Improve circulation pattern that will allow longer bubble path for longer time for mass transfer to occur.

Give oxygen where it’s needed most and less oxygen where the demand for oxygen is less.

Increase the efficiency of solid removals upstream in wastewater treatment to reduce the organic load and oxygen demand downstream.