Coagulation - Flocculation
In coagulation operations, a chemical substance is added to an organic colloidal suspension to cause its destabilization by the reduction of forces that keep them apart. It involves the reduction of surface charges responsible for particle repulsions. This reduction in charge causes flocculation (agglomeration). Particles of larger size are then settled and clarified effluent is obtained. A diagram of a coagulation-flocculation and settling of a wastewater is shown in Figure 5.1.
In fisheries wastewaters, the colloids present are of organic nature and are stabilized by layers of ions that result in particles with the same surface charge, thereby increasing their mutual repulsion and stabilization of the colloidal suspension. This kind of wastewater may contain appreciable amounts of proteins and micro-organisms which become charged due to the ionization of carboxyl and amino groups or their constituent amino acids. The grease and oil particles, which are normally neutral, become charged due to preferential absorption of anions (mainly hydroxyl ions). The parameter that characterizes the stability of a particle is called the Z potential and is a measure of the potential that would be required for destabilization of the particles. However, the usefulness of the Z potential is often questioned since it varies with the composition of the solution and is not repeatable.
Figure 5.1. Simplified diagram of a chemical coagulation process
The coagulation processes generally employ several steps. First, coagulant is added to the effluent, and mixing proceeds rapidly and with high intensity. The objective is to obtain intimate mixing of the coagulant with the wastewater, increasing the effectiveness of destabilization of particles and initiating coagulation. A second stage follows in which flocculation occurs for periods of up to 30 minutes. In the latter case, the suspension is stirred slowly to increase the possibility of contact between coagulating particles and to facilitate the development of large flocs. These flocs are then transferred to a clarification basin in which they settle and are removed from the bottom while the clarified effluent overflows.
Several substances may be used as coagulants. Given the proteinaceous nature of several wastewaters, pH can be adjusted by adding acid or alkali. The first is more common and causes coagulation of the proteins by denaturing them, changing their structural conformation due to the change in charge distribution on their surface. Thermal denaturation of proteins can also be used but due to its high energy demand it is only advisable if excess steam is available. In fact, the "cooking" of the blood- water in fishmeal plants is basically a thermal coagulation process.
Polyelectrolites are also commonly used as coagulants. They act as coagulants by lowering the charge of the wastewater particles. For this case, cationic polyelectrolites are preferred since wastewater particles generally have a negative charge. Some polyelectrolytes act as "bridges" between the already- formed particles. In the flocculation process, bridged particles interact with other bridged particles and this results in the increase of floc size. For this purpose, anionic or neutral polyelectrolites are used.
Since the recovered sludges from coagulation-flocculation processes may sometimes be added in the formulation of animal feeds, it is advisable to check that the coagulant or flocculant used are not toxic.
In fisheries wastewaters there are several reports of the use (at both pilot plant and at working scale) of inorganic coagulants such as aluminum sulphate and ferric chloride or ferric sulphate as well as organic coagulants (Johnson, 1984; Nishide 1976 and 1977; Ziminska, 1985).
There are reports (Hood and Zall, 1980) that fish scales can be used effectively as an organic wastewater coagulant. For this purpose, they are dried and ground before being added as coagulant in powder form. Another by-product of marine origin that is used as coagulant is chitosan, a natural polymer derived from chitin, a main constituent of the exoskeletons of crustacea.
No single set of operating conditions will satisfy all kinds of fisheries wastewaters and, therefore, the operational policy, pH, and coagulant dosage must be determined for each individual case. This is generally done in the laboratory using beakers and stirrers, trying to simulate the operation at full scale: first, the coagulant is added and rapidly mixed, then stirring proceeds slowly to allow flocculation. Since several variables are studied at a time, it is common to use multiple places (usually 4 or 6) stirrers. These allow constant stirring conditions to be maintained and therefore ascertain the effect of coagulant dosage and pH which is varied in the beakers. The device is commonly known as the "Jar Test apparatus" (see Figure 5.2).
Figure 5.2. Typical Jar Test apparatus, normally provided with a single speed control and indicator, and each stirrer can be either disengaged or placed at different heights
Chlorination is a process commonly used in both industrial and domestic wastewaters. Although there may be other reasons for its use in other kinds of wastewaters (e.g., cyanide oxidation), the reason for its use in fisheries effluents is the disinfection by destroying bacteria or algae or inhibiting their growth. Usually the effluents are chlorinated just before their final discharge to the receiving waterbodies.
For this process either chlorine gas or hypochlorite solutions may be used, the latter being easier to handle.
In water solutions, chlorine forms hypochlorous acid which in turn forms hypochlorite: Cl 2 + H2O « HOCI + H+ + Cl- HOCI « H+ + OCl-
A problem that may occur during chlorination of fisheries effluents is the formation of chloramines. These wastewaters may contain appreciable amounts of ammonia or volatile amines, which cause an increased demand of chlorine to achieve a desired degree of disinfection because they react with chlorine to give chloramines. The relative proportions of these products depends on the pH and concentration of ammonia and the organic amines present.
The degree of disinfection is attributed to the residual chlorine present in water. A typical plot of the residual chlorine versus the chlorine dosage is shown in Figure 5.3:
Figure 5.3. Curve of chlorine residual versus chlorine dosage
An initial amount of chlorine is reduced to chloride by the reducing agents present, and therefore the residual chlorine is negligible (segment a-b). Further addition of chlorine may result in the formation of chloramines. These appear as residual chlorine but in the form of combined chlorine residual (segment b- c). Once all the ammonia and organic amines, have reacted with the added chlorine, additional amounts of chlorine result in the disappearance of the chloramines by oxidation, with a decrease in the chlorine residual as a consequence (segment c-d). Once this oxidation is complete, further addition of chlorine results in the appearance of free available chlorine. The point d on the curve is also known as "breakpoint". The goal in obtaining some free chlorine residual is to ensure disinfection.
Chlorination units are generally simple, consisting of a chlorination vessel in which the wastewater and the chlorine are brought into contact. Sufficient mixing must be provided, and it is recommended that the residence time be not less than 30 seconds. Prior to final discharge, sufficient time (about 15 minutes) must be allowed for the chlorine to react. This may be done in the ducts which carry the wastewater to the discharge point, provided the residence time exceeds 15 minutes. Otherwise, a contact basin must be constructed. A typical configuration is shown in Figure 5.4.
Figure 5.4. Schematics of a chlorination system
The channels in this contact basin are usually narrow in order to increase the water velocity. This reduces accumulation of solids by settling. However, the space between the channels should allow for easy cleaning.
The levels of available chlorine after the breakpoint should comply with the local regulations that usually vary between 0.2 mg/l and 1 mg/l. This value strongly depends on where the wastewater is to be discharged because residual chlorine in wastewater effluents was identified in some cases as the main toxicant suppressing the diversity, size, and quantity of fish in receiving streams (Paller et al., 1983).
Although 15 minutes is very common, retention times of up to 30 minutes are also used. The chlorine dosage needed to achieve the residuals required will vary with the wastewater considered: 2-8 mg/l is common for an effluent from activated sludge plant, and can be as high as 40 mg/l in the case of septic wastewater (Eckenfelder, 1980, Metcalf and Eddy Inc., 1979).
Since ozone is also a strong oxidizing agent, it has been used for disinfection due to its bactericidal properties and its potential for removal of viruses. Ozone (O3) is produced when a high voltage is discharged across a narrow gap in the presence of air or oxygen. Ozonation systems generate O3 in situ. Figure 5.5 shows a diagram of an ozonation system.
Once ozone has been added and reacted, it reverts to oxygen increasing somewhat the dissolved oxygen level of the effluent to be discharged, which is beneficial to the eceiving water stream. Contact tanks are usually closed to recirculate the oxygen-enriched air to the ozonation unit. Advantages over chlorination are that it does not produce dissolved solids and is affected neither by ammonia compounds present nor by the pH value of the effluent. It is also used to oxidize ammonia and nitrites present in fish culture facilities (Monroe and Key, 1980).
Although much less used than chlorination in fisheries wastewaters, ozonation systems have been installed especially in discharges to sensitive waterbodies (Stover and Jarnis, 1979 Rosenthal and Kruner, 1985).
Figure 5.5. Diagram of an ozonation system