4. BIOLOGICAL TREATMENT


4.1 Aerobic Processes

4.1.1 Activated Sludge Systems
4.1.2 Aerated lagoons
4.1.3 Aeration
4.1.4 Trickling filters
4.1.5 Rotating biological contractors
4.1.6 Selection of aerobic treatments

4.2 Anaerobic Treatment

4.2.1 Digestion systems
4.2.2 Imhoff tanks


The goal of all biological wastewater treatment systems is to remove the non-settling solids and the dissolved organic load from the effluents by using microbial populations. Biological treatments are generally part of secondary treatment systems. The microorganisms used are responsible for the degradation of the organic matter and the stabilization of organic wastes. With regard to the way in which they utilize oxygen, they can be classified into aerobic (require oxygen for their metabolism), anaerobic (grow in absence of oxygen) and facultative (can proliferate either in absence or presence of oxygen although using different metabolic processes). Most of the micro-organisms present in wastewater treatment systems use the organic content of the wastewater as an energy source to grow, and are thus classified as heterotrophes from a nutritional point of view. The population active in a biological wastewater treatment is mixed, complex and interrelated. By example, in a single aerobic system, members of the genera Pseudomonas, Nocardia, Flavobacterium, Achromobacter and Zooglea may be present, together with filamentous organisms (Beggioata and Spaerotilus among others). In a well-functioning system, protozoas and rotifers are usually present and are useful in consuming dispersed bacteria or non-settling particles. More extensive description and treatment of the microbiology of wastewater treatment systems are given elsewhere (Stanier, 1976).

The organic load present is incorporated in part as biomass by the microbial populations, and almost all the rest is liberated as gas (carbon dioxide (CO2) if the treatment is aerobic, or carbon dioxide plus methane (CH4) if the process is anaerobic) and water. In fisheries wastewaters the non- biodegradable portion is very low.

Unless the cell mass formed during the biological treatment is removed from the wastewater (e.g., by sedimentation or other treatment described in the previous section), the treatment is largely incomplete, because the biomass itself will appear as organic load in the effluent and the only pollution reduction accomplished is that fraction liberated as gases.

The biological treatment processes used for wastewater treatment are broadly classified as aerobic (in which aerobic and facultative micro-organisms predominate) or anaerobic (which use anaerobic micro-organism.

If the micro-organisms are suspended in the wastewater during biological operation, the operations are "called suspended growth processes", while the micro-organisms that are attached to a surface over which they grow are called "attached growth processes".

This section explains the principles and main characteristics of the most common processes in each case.

4.1 Aerobic Processes

In these, the reactions occurring can be summarized as:

organic load + oxygen + more cells + CO2 + H2O

In fisheries wastewaters, the need for addition of nutrients (the most common being nitrogen and phosphorus) seldom appears, but an adequate provision of oxygen is essential for successful operation of the systems. The most common aerobic processes are: activated sludge systems, lagoons, trickling filters and rotating disk contactors. These aerobic processes are described, together with the devices used for aeration.

4.1.1 Activated Sludge Systems

These systems originated in England in the early 1900's and earned their name because a sludge (mass of microbes) is produced which aerobically degrades and stabilizes the organic load of a wastewater. Figure 4.1 shows the layout of a typical activated sludge system.

Figure 4.1. Diagram of a simple activated sludge system

For larger systems, especially when high variability is expected, the design involves the use of multiple aeration tanks and multiple settling tanks. The number of units employed depends on the flow of wastewater being generated.

The organic load (generally coming from primary treatment operations such as settling, screening or flotation) enters the reactor where the active microbial population (activated sludge) is present. The reactor must be continuously aerated. The mixture then passes to a secondary settling tank where the cells are settled. The treated wastewater is generally discharged after disinfection while the settled biomass is recycled in part to the aeration basin. The cells must be recycled in order to maintain sufficient biomass to degrade the organic load as quickly as possible. The amount that is recirculated depends on the need to obtain a high degradation rate and on the need for the bacteria to flocculate properly so that the secondary settling separates the cells satisfactorily. As the cells are retained longer in the system, the flocculating characteristics of the cells improve since they start to produce extra cellular slime which favours flocculating.

The most common types of activated sludge are the conventional and the continuous flow stiffed tank (Figure 4.1), in which the contents are completely mixed. In the conventional process, the wastewater is circulated along the aeration tank, with the flow being arranged by baffles in plug flow mode (Figure 4.2). The oxygen demand for this arrangement is maximum at the inlet as is the organic load concentration.

Figure 4.2: Diagram of a conventional activated sludge process

In the completely mixed process the inflow streams are usually introduced at several points to facilitate the homogeneity of the mixing; if the mixing is complete, the properties are constant throughout the reactor. This configuration is inherently more stable to perturbations because mixing causes the dilution of the incoming stream into the tank. In fisheries wastewaters the perturbations that may appear are peaks of concentration of organic load or flow peaks. The flow peaks can be damped in the primary treatment tanks. The conventional configurations would require less reactor volume if smooth plug flow could be assured, which usually does not occur.

Other versions of activated sludge systems (e.g., extended aeration, contact stabilization, step aeration and pure oxygen processes) are used in other kinds of wastewaters but are not known to be applied to treat fisheries wastewaters. They are discussed elsewhere (Metcalf and Eddy Inc., 1979; Eckenfelder, 1980).

In all activated sludge systems, the cells are separated from the liquid and partially returned to the system to have a relatively high concentration of cells that degrade the organic load in a relatively short time. Therefore two different resident times are characteristic: the hydraulic residence time (H) given by the ratio of reactor volume (V) to flow of wastewater (Q):

H = V/Q

and the cell residence time (c) given by the ratio of cells present in the reactor to the mass of cells wasted per day. Typical H values are in the order of 3-6 hours, while c fluctuates between 3 and 15 days. Such difference in resident times is obtained by discharging the clarified effluent but wasting only a small fraction of the sludge. This in turn can be accomplished by discarding a portion of the sludge from the settling tank or by wasting a fraction of the outlet of the reactor before entering the settling tank.

In activated sludge systems, organic load removals of 85-95% are the most common. A key factor in the success of these systems is its proper operation, which requires trained manpower. Although used by some large fisheries which operate on a year-round basis, activated sludge may not prove to be economical or feasible for small seafood processors who operate seasonally because of the need to have a fairly constant supply of wastewater to maintain the micro-organisms.

Problems may appear during the operation of activated sludge systems, including:

4.1.2 Aerated lagoons

The aerated lagoons are basins, normally excavated in earth and operated without solids recycling into the system. This is the major difference with respect to activated sludge systems. Two types are the most common: the completely mixed lagoon (also called completely suspended) in which the concentration of solids and dissolved oxygen are maintained fairly uniform and neither the incoming solids nor the biomass of microorganisms settle, and the facultative (aerobic-anaerobic or partially suspended) lagoons. In the facultative lagoons, the power input is reduced causing accumulation of solids in the bottom which undergo anaerobic decomposition, while the upper portions are maintained aerobic (Figure 4.3 gives an example). The main operational difference between these lagoons is the power input, which is in the order of 2.5-6 Watts per cubic metre (W/m3) for aerobic lagoons while the requirements for facultative lagoons are of 0.8-1 W/m3 . Being open to the atmosphere, the lagoons are exposed to low temperatures which can cause reduced biological activity and eventually the formation of ice. This can be partially alleviated by increasing the depth of the basin. These units require a secondary sedimentation unit, which in some cases can be a shallow basin excavated in earth, or onventional settling tanks can be used.

 

Figure 4.3. Diagram of aerobic (top) and facultative (bottom) aerated lagoons

If excavated basins are used for settling, care should be taken to provide a residence time long enough for the solids to settle, and there should also be provision for the accumulation of sludge. There is a very high possibility of offensive odour development due to the decomposition of the settled sludge, and algae might develop in the upper layers contributing to an increased content of suspended solids in the effluent. Odours can be minimized by using minimum depths of up to 2 m, while algae production is reduced with liquid retention time of less than two days.

The solids will also accumulate, all along the aeration basins in the facultative lagoons and even in comers, or between aeration units in the completely mixed lagoon. These accumulated solids will, on the whole, decompose in the bottom, but since there is always a non-biodegradable fraction, a permanent deposit will build up. Therefore, periodic removal of these accumulated solids becomes necessary. 

4.1.3 Aeration

The aerated systems described above need an oxygen supply. Depending on the characteristics of the process, different designs may be used. The oxygen can be supplied to the activated sludge by either diffused aeration, by turbine agitation, by static aerators, or by surface coarse or large bubble diffusers. The last two are used also in the lagoon systems.

The diffused aeration systems (Figure 4.4) are also divided into fine bubble, medium and coarse or large bubble diffusers. The fine bubble diffusers are built of porous materials (grains of pure silica or aluminum oxide are bonded ceramically or by resins) which provide very small bubbles of high surface area that favour the oxygen transfer from the air to the wastewater. The medium bubble diffusers are perforated pipes or tubes wrapped with plastic or woven fabric. The coarse or large bubble diffusers can be orifice devices of various types, some of which are designed to be non-clogging (Figure 4.5).

 

Figure 4.4. Fine and medium bubble diffusers

 

 

Figure 4.5. Coarse or large bubble diffusers

With the small or fine bubble diffusers, it is important to use air free of particles that would otherwise clog them. Although somewhat less efficient for oxygen transfer, the coarse bubble diffusers are sometimes preferred because the presence of particles in the air is not a critical problem, and also for their lower cost and maintenance requirements. The diffusers are placed along air manifolds, close to the bottom of the aeration tanks.

The static aerators (Figure 4.6) are vertical tubes placed at the bottom of the aeration tank, with packing material along its length. The compressed air is supplied from the bottom of the tubes, forcing a mixture of air and water through the packing, where most of the oxygen transfer to the wastewater takes place. They have been used mainly in aerated lagoons.

 

Figure 4.6. Sketch of a static aeration system

The turbine aerators are one of the most common and simple aeration devices and consist of an electric motor-driven turbine impeller rotating at high speed above a pipe or a sparging ring which discharges the compressed air (Figure 4.7). The air bubbles discharged from the pipes are dispersed by the rotation of the turbine. Depending on the depth of the aeration basin, more than one impeller may be used in the same axis . The power drawn by the turbine systems is used for maintaining the micro- organisms in suspension and to break down and disperse the air bubbles, the latter demanding most of the power.

 

Figure 4.7. Turbine aeration system

The most common surface aeration units (Figure 4.8) are mounted on a float and consist of a propeller installed inside a rising tube and driven by a non-immersed motor. The propeller draws the liquid from under the unit and sprays it above the surface of the tank. The oxygen transfer takes place from the air to the droplets sprayed and to the turbulent surface of the liquid surrounding these units. Other surface aeration units are the so-called "brush" aerators which are basically blades mounted on a cylinder which rotates through the liquid (Figure 4.9). Usually, these units require baffles to direct the flow and insure turbulent velocity.

Figure 4.8. Diagram of a floating surface aerator

Figure 4.9. Sketch of a surface brush aerator

The oxygen transfer rate of the different devices fluctuate between 0.7 and 1.4 kg of oxygen per kiloWatt-hour when used in actual wastewaters. Most catalogues give much higher transfer capacities, because these values are based on test under standard conditions (typically clean, tap water at 20C and no dissolved oxygen at the start of the test). When selecting aeration equipment, care should be taken in interpreting these values and transfer rates in actual wastewater should be requested for proper evaluation.

4.1.4 Trickling filters

The trickling filter is one of the most common attached growth processes. Rather than being suspended as in activated sludge or aerated lagoons, most of the biomass is attached to some support media over which they grow (Figure 4.10).

 

Figure 4.10. Cross-section of an attached growth biomass film

The organic contents of the effluents are degraded by the attached growth population which absorbs these organic contents from the surrounding water film. Oxygen from the air diffuses through this liquid film and enters the biomass. As this organic matter grows, the biomass layer becomes thicker and eventually some of the inner portions of the biomass will be deprived of oxygen or nutrients and will separate from the support media over which a new layer will start to grow. The separation of biomass occurs in relatively large flocs which settle relatively quickly compared with suspended cells. Air circulates between the interstitial spaces of the supporting material. The media that can be used are beds of rocks (ranging in size from 5 to 10 cm) randomly packed, although regular packings of plastic material (Figure 4.11) are becoming more common recently in view of its much lighter weight, better flow distribution, larger void space and specific area.

 

Figure 4. 11. Typical packing for trickling filters

The trickling filter units consist of a circular tank filled with the packing media in depths from 1 to 2.5 m, or 10 m if synthetic packing is used. The bottom of the tank must be constructed rigid enough to support the packing and also designed to collect the treated wastewater which is either sprayed by regularly-spaced nozzles or (more common) by rotating distribution arms (Figure 4.12). The liquid percolates through the packing and the organic load is absorbed and degraded by the biomass while the liquid drains to the bottom where it is collected.

 

Figure 4.12. Sketch of a trickling filter unit

With regard to the packing over which the biomass grows, the void fraction and the specific surface area are important features; the first is necessary to ensure a good circulation of air and the second to accommodate as much biomass as possible to degrade the organic load of the wastewaters. Although initially more costly, the synthetic packings have larger void space, larger specific area and are lighter. Usually, the air circulates naturally, but in some high-strength wastewaters forced ventilation is used. They can be used with or without recirculation of the liquid after the settling tank. The need for recirculation is dictated by the strength of the wastewater and the rate of oxygen transfer to the biomass. Typically, recirculation is used when the BOD5 of the wastewater to be treated exceeds 500 mg/litre.

As with all biological systems, low temperatures reduce the degrading capacity of trickling filters. In cold areas trickling filters may be covered.

The BOD5 removal efficiency varies with the organic load imposed but usually fluctuates between 45 and 70% for a single-stage filter. Removal efficiencies of up to 90% can be achieved in two stages.

4.1.5 Roating biological contractors

Rotating biological contractors (RBC) units are another form of attached growth processes. In RBC units the biomass is attached to disks (up to 3.5 m in diameter) which rotate at 1 to 3 rpm while immersed up to 40% in the wastewater (Figure 4.13). The disks are made of corrugated, light plastic material.

 

Figure 4.13. Diagram of rotating biological contractor (RBC) unit

When exposed to air the attached biomass absorbs air and when immersed the microorganisms absorb the organic load. A biomass of 1-4 mm grows on the surface and its excess is teared off the disks by shearing forces and is separated from the liquid in the secondary settling tank. A small portion of the biomass remains suspended in the liquid within the basin and is also responsible in minor part for the organic load removal. Rotation speeds of more than 3 rpm are seldom used because this increases electric power consumption while the oxygen transfer does not increase sufficiently. The ratio of surface area of disks to liquid volume is typically 5 l/m2. For high-strength wastewaters, more than one unit in series (staging) is used. The effect of lower temperatures is partially mitigated by the use of housing for the disk units. These systems are normally operated without recycling the liquid. The power consumption is in the order of 2 kW/1 000 m3/day of capacity. They have been used to upgrade activated sludge existing plants, placing the disk units in the aeration basins (Antonie, 1978).

4.1.6 Selection of aerobic treatments

Several factors (apart from the economics) influence the choice of a particular aerobic treatment system. There is no universal solution and the decision of which system to use (or even if using an aerated system or not) depends on many aspects. Key factors are: the area available, which sometimes is the deciding aspect; the ability to operate intermittently is critical for several fishing industries which do not operate in a continuous fashion or work only seasonally; the skill needed for operation of a particular treatment cannot be neglected; and finally the costs (both operating and initial investment) are also sometimes decisive. The following table summarizes these factors when applied to aerobic treatment processes:

Table 4.1. Factors affecting the choice of aerobic processes

(a) OPERATING CHARACTERISTICS
System Resistance to shock loads of organics or toxics Sensitivity to intermittent operations Degree of skill needed
Lagoons

Maximum

Minimum

Minimum

Trickling filters

Moderate

Moderate

Moderate

Activated

Minimum

Maximum

Maximum

(b) COST CONSIDERATIONS
System

Land needed

Initial costs

Operating costs

Lagoons

Maximum

Minimum

Minimum

Trickling filters

Moderate

Moderate

Moderate

Activated

Minimum

Maximum

Maximum

Adapted from Rich, 1980.

The considerations for the RBC systems are similar to those of trickling filters.

4.2 Anaerobic Treatment

The anaerobic treatment of wastewater proceeds with degradation of the organic load to gaseous products (mainly methane and carbon dioxide) which constitute most of the reaction products and biomass. Anaerobic treatment is the result of several reactions: the organic load present in the wastewater is first converted to soluble organic material which in turn is consumed by acid producing bacteria to give volatile fatty acids, plus carbon dioxide and hydrogen. The methane producing bacteria consume these to produce methane and carbon dioxide. This is summarized in Figure 4.14.

Figure 4.14. Scheme of reactions produced during anaerobic treatment

These processes are reported to be better applied to high-strength wastewaters (e.g., blood water or stickwater).

4.2.1 Digestion systems

A typical diagram is shown in Figure 4.15, which presents an anaerobic system.

Figure 4.15. Diagram of an anaerobic digestion process

The flow resembles that of an activated sludge process except that anaerobic digestion occurs due to absence of oxygen. Good sealing of the digesting tanks is essential since oxygen kills some of the anaerobic bacteria present and air presence may easily disrupt the process. From the anaerobic digester the effluent proceeds to a degasifier and to a settler from which the wastewater is discharged and the solids are recycled. The need for recycling appears from the fact that anaerobic digestion proceeds at a much slower rate than aerobic processes, thereby requiring more time and more biomass to achieve high removal efficiencies.

The anaerobic processes have been applied in fisheries wastewaters, obtaining high removal efficiencies (75-80%) with loads of 3 or 4 kg of COD/day/m3 of digester (Balslev-Olesen et al., 1990; and Mendez et al, 1990).

An alternative process employs a treatment tank filled with packing on which the wastewater is circulated. The bacteria responsible for the anaerobic digestion growth attach on the surface of the packing.

The gas produced by a balanced and well-functioning system contains 60-70% of methane, the rest being mostly carbon dioxide and minor amounts of nitrogen and hydrogen.

Anaerobic processes are also sensitive to temperature. This is why in some cases heating is provided to the digester to reach temperatures of 30-35C. In most cases this can be done in part with the methane gas originating from the digester.

4.2.2 Imhoff tanks

The Imhoff tank is a relatively simple system used originally instead of heated digesters. It is still used for plants of small capacities. It consists basically of a two-chamber rectangular tank, usually built partially underground (Figure 4.16).

Figure 4.16: Section of an Imhoff tank

The wastewater enters in the upper compartment which acts as a settling basin while in the lower part the settled solids are. stabilized anaerobically.

Short-circuiting of the wastewater can be prevented by using a baffle at the entrance together with more than one port for discharge.

The lower compartment is generally unheated. The stabilized sludge is generally removed from the bottom generally twice a year to give ample time for the sludge to stabilize, although the removal frequency is sometimes dictated by the convenience of sludge disposal. In some cases, these tanks are designed with inlets and outlets at both ends, and the flow of wastewater is reversed periodically so that the sludge in the bottom accumulates evenly. Although they are simple installations, they are not without inconveniences: foaming, odour and scum formation. These typically result when temperature falls below 15C, when the bacteria that produce volatile acids (see Figure 4.14) predominate and methane production is reduced causing a process imbalance. This is why in some cases immersion heaters are used during cold weather. The scum forms because the gases that originate during the anaerobic digestion are entrapped by the solids and have no time to escape from the solids causing them to float. This is usually overcome by increasing the depth in the lower (digestion) chamber. At lower depths, bubbles form at a higher pressure, expand more when rising and are more likely to escape from the solids. The odour problems are minimal when the two stages of the process (acid formation and gas formation) are balanced.