6. Presentación de datos
6.2. Reflexión final de la presentación de datos: Cuñas motrices
large numbers of filamentous fungi in a plant leads to the problem of sludge 'bulking', a problem
where the specific gravity of the sludge is reduced
toa point where gravity settling is very difficult
(Grady and Lim,1980). This may be caused by a number of factors such as a low pH, the addition of
toxic compounds, undcrloading or overloading (Tomlinson and Williams, l 975). Geotrichum is often
responsible for this problem (Tomlinson and Williams,1975).
2.3.2.3 Protozoa and Rotifers. Protozoa
3 1
These are small, multi- or uni-cellular eukaryotic organisms that are frequently present in activated sludge (Greenfield,1987). They are motile, and generally 10 times larger than bacteria (Greenfield,1987). Ciliated organisms are more likely to be present in an established system than the flagellated species (Curds,1975). The majority are aerobic heterotrophs (Greenfield,1987), and are sensitive to low dissolved oxygen, pH variations and large amounts of carbon dioxide (Curds,1975). Their role is to add a "final polish" to the effluent by reducing the BOD, which microbial cells represent. They consume free swimming bacteria, and particulate organic matter, hence reducing the amount of material that will not be removed by gravity settling (Greenfield,l987). The presence of these organisms in activated sludge can be used as an indicator of the operation of the plant and effluent quality.
Rotifers
Rotifers are aerobic multicellular animals, usually possessing a foot for attachment, and two sets of rotating cilia for motility and catching food (Doohan,1975).
These organisms also prey on bacteria, both dispersed and flocculated, and degrade particulate organics (Greenfield,1987). By breaking up floes, they provide nuclei for further floc formation, as well as clearing the effluent by removing free swimming organisms (Doohan,1975). The presence of rotifers is thought to indicate a highly efficient purification process (Greenfield,1987).
2.3.3 Process 2.3.3.1 Toxic
The effects of toxic compounds on the oxidative efficiency of activated sludge plants are exactly the same as would occur in any active biological process faced with an inhibitor. In the case of activated sludge, the inhibitors fall into two classes: inorganic and organic.
This class contains boron and the heavy metals such as chromium, copper, lead, silver and arsenic. It also includes anions such as cyanides and chromates present in some industrial wastes (Tchobanoglous,1979). If such compounds are present in concentrations sufficiently high to cause inhibition, then they must be physically or chemically removed prior to biological treatment. Care must also
be
taken to ensure that toxic concentrations do not bioaccumulate in the sludge, and cause sludge disposal problems (Melcer,1987).Inhibitors.
At first glance, the presence of high concentrations of a toxic organic chemical would appear to preclude the use of a biological treatment system. As the waste to be considered here contains high concentrations of chlorophenols, this appears to eliminate the use of activated sludge as a treatment
sludge, but does affect process configuration selection.
According to current thinking plug flow systems, and their various modifications cannot be used for toxic organics, because the presence of the such compounds will reduce the overall rate of removal to values much lower than can be achieved by alternative configurations. This reduction can be explained in term of inactivation of the returned sludge due to exposure to the fresh incoming waste, and increasing the volume requirements of the system (Grady and Lim,l980). For this reason plug flow systems are not used for toxic compounds.
A recent theoretical study by Santiago and Grady (1989) has suggested that plug flow systems are more stable than expected. These authors characterise the inhibitory nature of a compound by the ratio of K1 to K5, as the
p
value. It is suggested that plug flow systems will perform better than a CSTR to a shock load of a mild inhibitorCP
= 5), but will be more sensitive to subsequent shock loads. Based on the phenol data of Rozich and Gaudy (1984), phenol is four times more inhibitory than the mild inhibitor of Santiago and Grady (1989), with ap
value of 1 .05. The data of Tyler and Finn ( 1974) however, indicates ap
value of 3.05 for 2,4-DCP, implying it is less inhibitory than phenol, although the K1 for 2,4-DCP is 35.7 mg/1 c.f. 62 mg/1 for phenol. As it appears that 2,4-DCP would be more inhibitory than phenol, there are grounds for questioning this approach.A second flaw in the study of Santiago and Grady (1989) is the assumption that the inhibition is reversible. The toxic nature of the chlorophenols, and their ability to attack DNA (Kleist-Welch G uerra and Lochmann,l988) suggests such an assumption may be invalid. As it appears two of the assumptions may be violated, it would be wise to use a more conservative approach and choose the CSTR approach.
For the reasons outlined above, this thesis is concerned with the oxidation of toxic compounds and hence further discussion will be limited to CSTR approaches.
CSTR systems are the most widely used AS systems for the degradation of toxic compounds (Me leer, 1987). This is because the aeration basin provides equalisation facilities in addition to the treaunent (Grady and Lim,1980). The nature of CS1R systems is such that newly introduced toxic compounds are immediately dispersed throughout the aeration basin, and hence do not reach concentrations high enough to warrant concern.
Another recent development (Melcer,1987) is the introduction of the powdered activated carbon treaunent process (PACT). This process involves the addition of small quantities of activated carbon to the aeration basin, which serves as a support for microbial growth, and as a matrix for organic compounds prior to their oxidation by the organisms present (Melcer,1987). Other claimed benefits include better sludge settling and greater stability to shock loads (Melcer,1987). Work has shown that this addition (at approximately lOOmg/1 activated carbon) does not assist the removal of readily
3 3 degraded compounds such as benzene and toluene, but does assist in the removal of the more recalcitrant molecules such as 1 ,2-dichlorobcnzene and 1 ,2,4-trichlorobenzene (Melcer,1987).
2.3.3.2 Control of Mean Cell Residence Time.
The mean cell residence time (MCRT) is probably the most important control parameter in an activated sludge plant, especially one treating inhibitory compounds (Lange � al.,1989). The main factor affecting the control of the MCRT is the ease of separation of the biomass from the treated waste (Grady and Lim,1980). The most common method of achieving cell separation in activated sludge plants is gravity settling (Tchobanoglous,1979). However, effective settling can only be achieved when the organisms have flocculated into particles large enough to settle rapidly (Grady and Lim,1980).
It has been observed that flocculation only occurs between two threshold values of MCRT, a lower value of about 2 days, below which dispersed growth makes settling difficult, and an upper limit of 15 days, above which endogenous respiration causes the floes to break up and become dispersed pin floes (Grady and Lim,1980).
Both of these problems have the same effects: the inability to produce a good, well thickened sludge for recycle and wasting, and also to allow large amounts of the biomass to be lost in the effluent, decreasing its quality.
Despite the pin floc problem at high MCRT's, such times are often required for industrial wastewaters (Grady and Lim,1980). As a result of this, it is important to study sludge settling characteristics during laboratory scale testing of such processes.
Clarifier design plays an important role in the control of MCRT. For activated sludge use, the clarifier must perform three functions:
(1) allow the flocculation of the bacteria to occur under conditions of low shear,
(2) allow sufficient time for settling to occur under conditions that do not cause the floes to break up,
(3) allow the thickening of the sludge to a point where the recycle ratio does not require an excessively large aeration basin (Tchobanoglous,1979).
A typical clarifier used with activated sludge plants is shown in Figure 2.13. The circular system is based on the influent mixed liquor being introduced to the centre of the clarifier, arid the clarified effluent being removed at the edges via a weir system. The tank bottoms are also scraped and slope to the centre to encourage the movement of sludge to the drawoff point. Drawoff is usually by vacuum (Eckenfelder,1980).
The thickening of the sludge is also important to maintain the MCRT. If the sludge was not thickened appreciably, then the recycle ratio would be increased to the point where the recycle flow
S l udge draw-off p i p e
S c r a p e r b l a des
Adj u s t a b l e s q u e e g e e s
2. 13: Typical Clarifier Used in Activated Sludge Systems (from Eckenfelder,l980)
G)
( rim t a ke-o ff)
E ffl u e n t p i pe
3 5
from the clarifier was much greater than the waste flow i e a recycle ratio much greater than 1 00%. S uch high recycle ratios, while not operationally impossible to maintain, are almost never used in practice.
The main operational problem which may affect the settling and thickening of the sludge (and hence the MCRT) is bulking, where the presence of filamentous bacteria and fungi produces light, 'fluffy' floes which do not settle well, resulting in poor effluent quality (Pike,l975). As stated earlier, bulking can be caused by a number of factors, and to solve a bulking problem or any given plant will require a detailed analysis of the mixed liquor and plant loading rate (Pike, 1975).
With these criteria satisfied, it is possible to run an effective AS plant. Varying the MCRT is an effective means of coping with variation in feed quality, temperature and other factors providing the variations are relatively slow (Grady and Lim,1980). Whilst automatic control of the major parameters is not difficult (Vaccari � al.,1988), process control with rapid changes in influent conditions is much more difficult than if physical or chemical treatment were used (Grady and Lim,1980).
Sludge disposal needs to be mentioned as wasted sludge is the largest by-product of the activated sludge process. As this thesis is not concerned principally with the disposal of sludge, a brief summary of a recent review of the topic will be provided for completeness.
Sludge for further treatment typically has a moisture content of 94 to 98 % water (Bogoni, 1988), and the first step of the disposal process is to reduce the moisture to give a solids content of up to 15 % . There are three main methods of disposal of sludge:
( 1 ) Anaerobic digestion of the sludge to produce methane and a small quantity of digested sludge which is then easily dewatered or incinerated (Bogoni,1988). This process is widely used, although the size of systems and control difficulties are a problem (Bogoni, 1988).