Fundamentals of Microbiology

Lesson 32. WASTE WATER TREATMENT AND DISPOSAL

Module 7. Environmental microbiology

Lesson 32

WASTE WATER TREATMENT AND DISPOSAL

32.1 Introduction

Waters become polluted as a result of domestic and industrial sewage disposal into surface waters, which contain huge amounts of various compounds that affect the biocenosis of water reservoirs. Besides sewage, pollution is also caused by rain run-offs which wash away different fertilizers and crop protection products. Moreover, the pollutants also transfer into waters from the surrounding air. This usually results from industrial dust which falls directly into the water or is washed away from the ground surface by rain. Important gases are: sulphur dioxide, nitrogen oxides, carbon oxides and dioxides which get into the waters mainly in highly industrial areas. Some of the above compounds undergo microbiological decomposition relatively easily, becoming food for heterotrophic microorganisms, others are resistant to such decomposition and are harmful or toxic to microorganisms. Examples of these are the following: cyclic compounds, engine oil, lubricants, chlorinated hydrocarbons, pesticides, and among the mineral pollutants – heavy metal salts.

32.2 Self-Purification of Surface Waters

Self-purification encompasses complex co-operation between physical and biochemical factors such as: sedimentation (settling), oxidation, an exchange of volatile substances between the atmosphere and water, and the release of gaseous products of metabolism into the atmosphere. However, the critical role is played by the biological factors.

A wide range of microorganisms and higher organisms participate in self-purification processes. Bacteria and fungi are the most crucial as they are capable of mineralising various mineral components. Proteins, simple and complex sugars, fats, cellulose, lignin, wax and others undergo degradation during the process of self-purification. As a result of mineralization the following compounds are created: H2O, CO2, NO3-, SO42-, PO43-, and other simple compounds. With the progression of self-purification the populations of microorganisms that act in the environment change (Fig. 32.1).

Fig. 32.1 Succession of microorganisms during the self-purification process

The self-purification process utilizes large amounts of oxygen during the biochemical processes. The amount of oxygen that is used up in any specified time by water microorganisms is called the biochemical oxygen demand (BOD). By analyzing the BOD, it is possible to determine the concentration of the organic compounds dissolved in water which are susceptible to biological oxidation. The discharge of impurities into the water reservoir creates a sudden change in chemical, biological and physical conditions. Simultaneously, right below the area of the discharge, the process of self-purification begins. The process leads to the formation of zones containing characteristically gradually decreasing levels of pollution.

32.3 Types of Sewage

• Domestic sewage contains large amounts of faecal matter, plant and animal wastes, surface-active agents, urea. The sewage comes from households, public lavatories and industrial facilities posing a serious hygienic and epidemiological threat,
• Industrial (technological), evolve during all types of industrial processes (manufacturing and processing),
• Precipitation (rain and melt waters) contain various atmospheric impurities (dusts, microorganisms, gaseous substances), surface run-offs, streets and paved surfaces run-offs (oils, liquid fuels, bacteria, small particle suspensions), microbiological impurities (bacteria, viruses, fungi).

32.4 Methods of the Wastewater Treatment

Depending on the type of pollutants, there are different methods of purification used prior to reintroduction into a receiving body of water. The methods are classified as follows:

• Mechanical – in this method only non-soluble pollutants are removed by utilizing the following processes: gravitational and centrifugal sedimentation, flotation, source filtration, separation in hydrocyclones, which allow the removal of organic and mineral suspensions as well as floating bodies;
• Physical-chemical – utilizes the following operations and processes: coagulation, coprecipitation, sorption, ion exchange, electrolysis, reverse osmosis, ultrafiltration;
• Chemical – utilizes neutralization, oxidation, reduction;
• Biological – consists of sewage purification (elimination of organic pollutants as well as biogenic and some refraction compounds) during biochemical processes of mineralization conducted naturally by microorganisms in a water habitat (e.g. sprinkling of wastewater onto agricultural lands), or in special devices (on trickling filters or activated sludge).

32.5 Stages of Sewage Treatment

A typical process of sewage treatment consists of four stages of purification:

• Mechanical (stage I of purification),
• Biological (stage II of purification),
• Elimination of biogenic compounds (stage III of purification),
• Water renovation (stage IV of purification).

Stage I of purification, primary treatment the so-called initial or mechanical purification. The goal of this stage is the removal of solid impurities. This stage is considered to be the preparation of sewage for further purification. By utilizing simple mechanical operations the following impurities are removed during the first stage:

• floating solid impurities,
• settling suspensions,
• oils and fats.

Stage II of purification, secondary treatment includes biological purification, which leads to the biodegradation of soluble organic impurities, colloidal systems and suspensions not removed during the first stage. The intensification of purification processes is obtained by utilizing trickling filters and activated sludge.

Stage III of purification, tertiary treatment includes processes used to thoroughly clean sewage. The largest impurities removed during this process are the biogenic compounds. The nitrogenous compounds are removed during the process of biological nitrification and de-nitrification, whereas the compounds of phosphorus are eliminated by a process of chemical precipitation. The role of thorough cleaning of sewage in this stage is the prevention of water eutrophication.

Stage IV of purification (water renovation) includes the processes of residual sewage removal, which are left over from the previous stages of purification. Water regeneration involves a set of methods which confer the properties of natural water onto the sewage so that it can be utilized in industrial facilities. Water regeneration allows the recycling of sewage, which is a significant element in water resource management, especially in regions low in water. There are several systems of water regeneration, from very simple ones, that use rapid filters or straining through microsieves, to very complex physico-chemical processes: coagulation, membrane processes and disinfection, sedimentation, expelling of ammonium, recarbonization, absorption, ion exchange, and water demineralization.

32.6 Techniques Used for Wastewater Treatment

32.6.1 Use of trickling filters (Fig. 32.2 & 32.3)

The treatment of sewage by trickling filters is conducted in reservoirs filled with loose, grainy and porous material. Sewage is sprayed upon the upper layer of the bed with sprinklers and then left to seep through its content.

Fig. 32.2 Trickling filters

A mucous biological film forms upon the content of the bed. The film is composed of microorganisms such as: bacteria, protozoa and fungi. The role of the filter involves a constant supply of sewage and its flow through the trickling filter while maintaining contact with the biological film. During the flow, the sewage undergoes mineralization as a result of aerobic decomposition by microorganisms. The biological film is initially composed of zoogleal bacteria which produce mucous sheets. With time, the composition of species of the mucous membrane changes due to their succession. Besides bacteria, the following appear: fungi, protozoa, annelida and fly larvae. Depending on the amount of treated sewage, the trickling filters may be subdivided into percolating and flushing filters.

Fig. 32.3 Filter process

Depending on the amount of the organic load the following types of biofilters are distinguished

• Low loaded may be filled with natural or artificial material. The supplied organic material is less than 0.4 kg BZT5/m³×d. In percolating filters the film is more developed and the biological process of decomposition is almost complete. In the final phase of purification, intensive processes of nitrification occur, which lead to an increase of nitrates in a run-off to the secondary settling tank.
• Mid-loaded are filled with natural-synthetic material and work with a load between 0.4-0.65 kg BZT5/m³×d. In order to ensure an adequate concentration of the supplied sewage the recirculation of part of the purified sewage is utilized with this type of filters. The reduction of organic compounds upon these filters is adequate, and the processes of nitrification partially occur. The introduction of additional processes of purification is not necessary.
• High loaded (flushed) are filled with natural-synthetic material, the filter is loaded with: 0.65-1.6kg BZT5/m³×d. In flushing filters, the intensity of sewage flow is greater; however the biofilm is composed almost entirely of bacteria and does not develop as much as in the above stated case. Flowing sewage washes out used and dead biological material from the filter. The washed out material is transported in the form of flocy sediment. Only a partial mineralization of organic compounds occurs on that type of filter and the nitrification process is inhibited. A low content of nitrates in effluent from filters testifies to partial mineralization of organic compounds. In complex systems, after these types of filters, re-purification is utilized, as the quality of the purified sewage does not usually meet the required standards.

32.6.2 Activated sludge

The process of activated sludge relies on sewage purification by freely suspended matter. It consists of producing 50-100 μm flocs with highly developed surface areas. The floc is made up of brown or beige mineral nucleus, while on its surface it contains heterotrophic bacteria within the mucous envelopes. The method of activated sludge requires delivery of oxygen into the substrate for bio-oxidation of organic pollutants, which should be >0.5 mg/dm3 in order to ensure proper oxygen conditions for the bacteria.

32.6.2.1 Activated sludge characteristics

Activated sludge is a type of flocculent suspended matter created during the aeration of sewage. Treating sewage with activated sludge consists of mineralization of organic compounds, conducted mainly by bacteria and following the same biochemical processes as observed in self-purification. However, the speed of the process is much greater. These results from the fact that the conditions of intensive aeration, triggered during sewage flow through aeration tanks are conducive to the development of impurity-decomposing bacteria.

Agglomeraters (flocs) which consist of heterotrophic bacteria coagulated with mucous, form during the process of aeration in aeration tanks (flocculation). The floccules absorb impurities contained in sewage, whereas microorganisms in floc decompose the absorbed substances. Activated sludge has a spongy, loose structure, made of small openings of various shapes. Undisturbed floccules easily settle and thus allow the separation of the activated sludge from sewage.

Biocenosis of activated sludge is, for the most part, composed of heterotrophic bacteria.In small percentages - and only under particular conditions and in some arrangements - it's made up of chemolithotrophic bacteria, especially nitrifying bacteria. The most common species of activated sludge are: Zooglea ramigera, Pseudomonas fluorescens, Pseudomonas putida as well as bacteria of Achromobacter, Bacillus, Flavobacterium and Alcaligenes genera. The process of selection occurs naturally. The conditions in aeration tank, especially the chemical composition, pH-value and air conditions, are the determining factors for the diversity of the bacterial complex standards.

In unfavorable conditions (overloading of aeration tanks with easily available substrates, high oxygen deficit) excess development of flocs occurs causing the so-called active-sludge swelling. There are two distinguishable types of swelling: fibrous and non-fibrous swelling.

Fibrous swelling is caused by excess filiform bacteria (Sphaerotilus natans, Beggiatoa alba or Thiothrix nivea) or fungi development. Non-fibrous swelling is caused by bacterial development, which produces excess amounts of mucous.

Active sludge biocenosis is made up of not only bacteria but also protozoa, nematodes and rotifers. Even though these microorganisms do not play a major role, their presence is equally important. Protozoa feed upon bacterial cells forcing them to reproduce quickly, which essentially make them an important renewal and reactivating factor of the activated sludge.

The most common protozoa are: Vorticella, Carchesium and Opercularia as well as Anthophysa, Oxytricha,Stylonychia and Lionotus. There is an inverse relationship between flagellates and ciliates within activated sludge. While a large number of flagellates indicate an overload of sludge, the presence of ciliates goes to show it is functioning properly. During the course of sewage purification with activated sludge a characteristic succession of biocenosis is observed.

32.6.2.2 Activated sludge process (Fig. 32.4)

Sewage is directed to aeration tanks filled with activated sludge (thick suspension of microorganisms) after its mechanical purification. The content of the aeration tank is constantly aerated in order to provide an adequate amount of oxygen, to keep the activated sludge in a suspended state and to ensure its constant mixing. The aeration tank is a device, in which the development of the activated sludge results from continuous cultivation. There is a state of equilibrium between the rates of sewage inflow, concentration of nutrients, bacterial reproductive rate, and the rate of the sewage outflow containing some activated sludge in it. During the time of contact of sewage with the activated sludge, the decomposition processes occurring simultaneously enable the development of activated sludge biomass. Separation of purified sewage is done in a secondary settlement tank. Both sedimentation and clarification of the purified sewage, which is then carried off to a receiving body of water, occurs in the device. Activated sludge may be used again for purification; it is then recycled into the aeration chamber. However, quite often, before reuse, the sludge is directed to a regenerative chamber, where it is aerated in order to bring back its particular physiological properties. When the sludge collected in a secondary settling tank is not recycled, then, as an excess sludge, it is removed and subjected to additional processing.

Fig. 32.4 Activated sludge method

32.7 Chemical Wastewater Treatment

Purification of industrial sewage that contains mineral and organic compounds, and heavy metals, utilizes physical-chemical and chemical methods. They include the following processes: neutralization, coagulation, oxidation, reduction, sorption, flotation, membrane processes, extraction, electrolysis, distillation.

32.7.1 Neutralization

It is a process of chemical neutralization of sewage in relation to the pH. Depending on the make-up of sewage and the type of the reacting substance used, neutralization may be accompanied by a chemical process of precipitation and coprecipitation. Neutralization may be conducted by mixing acidic sewage with bases. Hydroxides are substances most often used in the process of neutralization: NaOH in the form of 20-30% solution, Ca(OH)₂ in the form of 5-15% milk of lime, Na₂CO₃ in solution form CaCO₃, MgCO₃, MgO, dolomite in the form of a grainy filter. Mineral acids are used for the neutralization of basic sewage: H₂SO₄, HCl, H₃PO₄ in the form of solutions as well as CO₂ in the form of a clear gas.

32.7.2 Coagulation

Coagulation is a process of binding colloidal particles and the suspension into clusters of particles called the agglomerates, which results in precipitation of the sediment in the form of coagulate. The factors which most often cause coagulation are: addition of an electrolyte solution to lower the electrolytic potential, addition of colloids of an opposite charge into the colloidal particles, creation of metal hydroxides that absorb ions, colloids and suspensions.

32.7.3 Oxidation

An oxidation process is conducted in order to remove organic compounds, non-organic compounds and microorganisms from sewage. The reacting substances used in oxidation are: chlorine, chlorine oxidizing compounds; NaOCl, CaOCl₂, chlorinated lime, chlorine dioxide, ozone.

32.7.4 Reduction

The process of reduction used in sewage purification mainly concerns chromium. Chromium salts (VI) are toxic, carcinogenic, bacteriocidal and are irritants to skin. Its bacteriocidal properties slow down the process of water self-purification.

Reduction of chrome from oxidation state of 6⁺ down to 3⁺ is conducted through reduction and precipitation of hydroxide, which belongs to a group of barely soluble compounds. Reduction is conducted either chemically or electrochemically.

32.7.5 Sorption

Sorption consists of binding liquid soluble substances to the surface of solids. Depending on the characteristics of the process it may be irreversible (chemiosorption), or reversible - adsorption. The characteristic of the process of sorption is determined by one of the components of force:

• Physical sorption - the result of van der Waals forces
• Chemical sorption - the result of valence forces
• Ion sorption - between groups of cations and anions in the structure of the substrate
• Sieve sorption - at the molecular level according to the mechanism of a molecular sieve

32.7.6 Flotation

A process of structural separation consisting of raising the hydrophobic impurities into the foam along with the rising gas bubbles. As a result, the foam formed has a much higher concentration of pollutants than the rest of the sewage.

32.7.7 Membrane processes

These processes consist of separation of particles by flowing through a porous layer (membrane). The following are the types of membrane processes: reversed osmosis, nanofiltration, ultrafiltration, electrodialysis.

32.7.8 Extraction

This consists of transfer of components from one phase of the solution into the second liquid phase (dissolvent). Consequently a solution of the component in the dissolvent is obtained. The required condition for the process is the presence of two liquid phases.

32.7.9 Electrolysis

It is the process in which electrical energy invokes chemical changes of the electrolyte. As a result of the electrical field the movement of ions toward the electrodes (upon which the process occurs) occurs

• Cathode Me⁺⁺ e^- → Me (reduction)
• Anode X^- → X + e^- (oxidation)

32.7.10 Distillation

Process that utilizes the difference between the composition of a liquid and vapor in the state of equilibrium.