Biological Clarification 122908

APPLICATION NOTE APPLICATION:

Biological Clarification

INDUSTRY DESC.:

Water & Wastewater Treatment

INDUSTRY SIC CODE: PRODUCTS: FAXBACK/WEB ADDRESS: Brian McMorris – XACT Sensing, Inc..

AUTHOR: REFERENCES:

Biological Clarification After mechanical cleaning, biological processes utilizing specific bacteria are used to purify wastewater. Aerobic and anaerobic are the two types of biological cleaning processes used. The following table shows consequences that arise from the two types of processes. Process Anaerobic Aerobic Operating costs Low

High

Volume

High

Low

Space required Low

High

Sludge loading High

Low

The inflow to community treatment plants is often much-diluted wastewater. Therefore they use mostly an aerobic process. An anaerobic process is often preferred in industrial clarifiers as these usually receive highly loaded and concentrated wastewater. A typical example is wastewater from paper and sugar factories. Unfortunately anaerobic processes are much more sensitive to changes in the quantity of wastewater inflow. An anaerobic cleaning process is recommended for wastewater with a chemical oxygen demand (COD) greater than 5000 mg per liter and one that must be precleaned to be subsequently taken to a conventional biological clarifier.

Microbiological Principles The aim of biological cleaning is to convert energy rich material with high molecular weight into material with less energy and having smaller molecules. Desirable end products are water and carbon dioxide. The bacteria that can carry out this conversion exist in community wastewater, but not in sufficient concentrations to be effective. This is especially true when difficult reactions such as nitrification should take place. Such reactions require specific microorganisms. These enter the wastewater as return sludge from the secondary clarifier. Returning the sludge in this way makes sure that the bacteria survive for a long enough time. This is clearly of vital importance for their reproduction. They will be able to reproduce and survive only in a medium that stays stable longer than their reproduction cycle The growth of bacteria is of course influenced by several factors. Typical of these are nutrient concentration, temperature and even the pH. The biological oxygen demand (BOD) indicates microbiological activity. Biological cleaning is equivalent to reducing the BOD. In normal practice the value of biological oxygen demand over five days (BOD5) is measured. 1. respiration in the substrate 2. endogenous respiration + respiration in the protozoa 3. Rest substrate + protozoa + endogenous respiration of not eliminated bacteria 4. starting nitrification due to NH+3-N

Factors Influencing Growth of Bacteria A BOD : N : P ratio of about 100 : 5 : 1 gives the best nutrient conditions for the growth of bacteria. There is usually a lack of BOD in community wastewater, whereas all other nutrients are present in sufficient quantities. Wastewater treatment is temperature dependent, a fact shown by the difference in performance between summer and winter operation. The following table shows the relevant figures for the way bacterium growth depends on temperature. Most bacteria grow best in liquor having a pH of 7.0 to 8.5. Problems may arise in liquor with pH values under 6.5. This can happen in plants with acidic industrial inflow that may inhibit nitrification. The performance of a wastewater treatment plant depends on the parameters that influence the reaction in the liquor. It also depends on retention time or space requirements and on the sludge loading. These can be determined at the planning stage whereas the former ones are operational. The residence time of wastewater in the activated sludge tank (retention time) is usually expressed in terms of space requirement there, that is, the BOD feed (biological oxygen demand) per unit volume and day. This is the quantity that directly determines the clarification capacity and the oxygenation needed for the activated sludge tank. The stress on the microorganisms depends on the relation of quantity of sludge to the BODfeed. Complete clarification can be achieved at sludge loading values of 0.3 g BOD per gram

dry matter day. At greater values the microorganisms may be overtaxed and achieve only a partial clarification. Temperature Max. Max. Sludge age (°C Nitrosomonas Nitrobacter min. dry matter (days 10°C

0.29

0.58

3.5

20°C

0.75

1.04

1.3

30°C

1.97

1.87

0.5

Anaerobic Cleaning In an installation with anaerobic cleaning the processes follow each other instead of being carried out in parallel. This means that external factors have a greater influence than in aerobic cleaning installations. The digestion of acetogen bacteria is some 30 % slower than that of chemo-autotrophs, therefore anaerobic processes are correspondingly slower than aerobic ones. Aerated pond clarifies have proved to be suitable in country districts serving up to 5000 population equivalents. especially as they have cost advantages. The aeration also stirs the pond. Limit switches regulate the oxygen content of the liquid. Typical values for oxygen content lie between 3 mg/I and 5 mg/I in such installations. Percolating filters are robust biological clarifiers and present no problems in operation. They are therefore preferred in small installations. The bacteria live in the filters and fall off them in a single body. This ensures good sedimentation properties in the next stage. Percolating filters naturally also have filtering properties. They are therefore well suited for the second stage of a biological installation. Disadvantages of percolating filters are the danger of their getting clogged if the pretreatment is inadequate. They are also a possible odor nuisance and are strongly dependent on the outside temperature. The most popular biological cleaning systems are activated sludge tanks. Their temperature dependence is small and their oxygen need is quickly adjusted to operating needs. For their proper functioning, an adequate measurement and regulating system must be installed. Fixed bed reactors have the advantages of both percolating filters and activated sludge tanks. Regular counter current or back washing can overcome their disadvantage of a certain tendency to clog. Just as with activated sludge tanks, operation of fixed bed reactors is flexible, especially when several are in series and can be regulated singly.

Anaerobic Process Users prefer fixed bed reactors for anaerobic wastewater treatment, because the biomass growth is slow and it is retained almost wholly in the system.

Nitrogen Elimination

To reduce the feeding of nutrients into underground or surface waters, operators should remove nitrogen from wastewater. They do this biologically using a kinetic enzyme process (transformation/reduction of nitraCes to elementary nitrogen) that reduces nitrates to simple nitrogen. In this reaction H+ ions are eliminated and therefore raise the pH that was reduced during nitrification. This process therefore helps the biological cleaning process. Several processes exist for the successful elimination of nitrogen. A common practice is to use the natural process of nitrification first and then denitrification. The problem arises that the cleaning process that occurs in the first place does not leave enough carbon for the metabolism of the chemo-organotrophic organisms (microorganisms (bacteria) that can denitrify sludge). The ratio of carbon to nitrogen as nitrate should be about three to one at the inlet of the denitrification zone. In installations having several stages the denitrification usually takes place in the first part of the second tank. A sufficient quantity of carbon must be provided here, either by reducing BOD (biological oxygen demand) to 50 % in the first stage or by partially by-passing the first tank. All processes need appropriate measurements and control arrangements. It is particularly important to measure the concentrations of oxygen, nitrates and ammonia.

Controlling the Process In normal practice only the oxygen supply is regulated in biological clarifiers. Control of the return sludge flow or the return water flow is rare. The return sludge flow is sometimes timed. Sometimes there is a ratio control between the incoming wastewater feeding and the return sludge flow. Measurement of the oxygen content and the pH are preconditions for controlling the oxygen supply. Membrane covered Clark cells with an oxygen content two-conductor or three-conductor system has proved to be suitable for measuring the oxygen content. The location of the measuring point should be chosen carefully taking the characteristics of the flow in the tank into account. There must be a control system in a biological clarifier with denitrification. This should measure and control the oxygen content and capture the nitrate and ammonia values on-line. This is economical in installations serving 20,000 population equivalents or more. Measuring the oxygen, nitrate and ammonia allows proper control of the biological stage and usually reduces operating costs. Nitrogen compounds (ammonium, nitrite, and nitrate) in sewage treatment plants are eliminated almost exclusively by means of biological processes. Microorganisms influenced by oxygen transfer decompose the nitrogen compounds into the elemental components, thereby rendering them harmless to the environment. A simple process recommendable particularly for smaller treatment plants is intermittent denitrification. Here the basin is first aerated (approx. 1.5-2.0 mg/l O2), causing nitrification to take place, i.e. the conversion of ammonia nitrogen to nitrate. The aeration is switched off after a defined period of time. When the oxygen dissolved in the activated sludge is used up, the microorganisms change their respiration and are then able to eliminate the nitrate. This denitrification phase continues until the nitrate is completely consumed, which is indicated by a step drop in the

redox voltage signal. The controller XR 35 N detects this "redox break point". Subsequently the basin is aerated once more, causing the cycle to restart with nitrification.

For More Information Contact: Brian McMorris XACT Sensing, Inc. Phone: 952-594-0474 Email: [email protected]