Decomposition Of Organic Matter And Ions In Water

Introduction

Organic matters are those matter whose source are the living organism. It is capable of decay and decomposition due to the action of heat and pressure and also by microbial actions. Decomposition of organic matter in water occurs mostly by microbial actions.

The main sources of organic matter in water are organic fertilizers; remain of microorganisms that are phytoplankton, zooplankton and benthos produced within the pond, faeces of the culture animals, and uneaten feed. Aquatic plants also play a major role in constitution of organic matter in water.

Organic matter decomposes rapidly, and its accumulation is not usually as great as often believed. More importantly, with proper management, problems associated with organic matter accumulation in ponds can be avoided.

Factors controlling decomposition

The process of microbial decomposition of organic matter is controlled by several factors which include water temperature, pH, concentration of dissolved oxygen, and chemical composition of the organic matter itself.

Temperature

Bacteria and other organisms of decay, decompose organic matter fastest between 30 to 35 degree temperatures. Doubling the temperature in the range of 0 to 35 degrees usually will double the rate of decomposition.

PH: pH is another major factor for decomposition of organic matter in water. pH is basically a measure of the acidity of water, too high or too low of it is not good. The optimal pH for the functioning and survival of microbial organisms is 7.5 to 8. When the pH is lower, decomposition by fungi is favoured over that by bacteria especially at pH less than 6. Fungi are not as efficient as bacteria in decomposing organic matter because they convert more of the organic matter to their own biomass than bacteria.

Chemical composition

Organic matter rich in Nitrogen are usually easier to decompose than organic matter of lower nitrogen content. One reason is that organic matter of higher nitrogen content contains less fibre, but an equally important reason is that microorganisms of decay need nitrogen to produce their cells (biomass). An organic residue or matter containing 3 or 4 present nitrogen may decompose several times faster than one with 0.5 to 2 present content of nitrogen. Of course, ammonia or nitrate nitrogen dissolved in the water can be used by microorganisms to decompose organic matter of low nitrogen content. In aquaculture ponds, organic matter usually is low in fiber content and high in nitrogen content, and if some of it is not, there usually is ammonia and nitrate nitrogen in the water.

Dissolved oxygen

One important factor that contributes to decomposition of organic matter in water is the abundance of oxygen. There are several milligrams per litre of dissolved oxygen (aerobic conditions), but at a depth of a few millimetres into the sediment, microbial activity depletes molecular oxygen (anaerobic conditions). Aerobic decomposition completely converts organic compounds to carbon dioxide and water.

However, most of the microorganisms that can decompose organic matter anaerobically do not completely convert organic compounds to carbon dioxide and water resulting in a larger amount of organic remains than present where decomposition is aerobic. However, there are other anaerobic bacteria that can use these partially decomposed organic residues and eventually break them down to carbon dioxide and water, but the process is slow. Thus, decomposition is favoured by presence of dissolved oxygen.

The ideal situation that favours the accumulation of organic matter in water bodies is low water temperature and pH, and the abundance of fibrous vegetation (aquatic plants) such as reed swamp plants that are of low nitrogen content. These are the reasons that peat bogs often develop in shallow marshes, especially in cold climates.

Decomposition rates

The components of the organic matter do not decompose at the same rate. Proteins, fats, and simple carbohydrate compounds decompose faster than fibrous components such as cellulose, lignin, tannins, and waxes. Most of the organic residue will decompose within a few weeks or months, but some of the material will persist for years. Moreover, the microorganisms of decay excrete organic compounds, and when they die, they become organic matter. The excretions of microorganisms and resistant remains of decomposing organic matter form large complex molecules of humic substances known as humus in terrestrial soils. Organic matter analogous to humus also accumulates in the sediment of water.

The rate of decomposition is governed by three sets of factors the physical environment (temperature, moisture), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself.

The water environment is as we all know very sublime. The sunlight reaches the top level of the water and cannot penetrate more than 1000 metres (provided that the depth of the water is more than 1000).

Temperature of water basically depends on other factor such as the availability of sunlight and the climate of the region and also the weather. The depth of the water where sunlight can reach has a higher temperature than the depth where it cannot.

The quality and quantity of the material available to decomposers is another major factor that influences the rate of decomposition. Water bodies rich in aquatic plants and animals and also the microbial organisms have a higher decomposition rate.

What is Natural Organic Matter

Natural organic matter or NOM is termed for the complex mixture of thousands of organic compounds found in water.

• These compounds are derived from decaying plant and animal matter.

Effects of Natural Organic Matter

From a drinking water perspective, our concern with natural organic matter is often focused on its interaction with treatment chemicals such as disinfectants and coagulants, as well as its fouling potential in filters and distribution systems.

Disinfectants such as chlorine can interact with Natural Organic Matter (NOM) in order to produce disinfection byproducts some of which are known carcinogens.

NOM can act as a food source for bacteria and contribute to biofilm growth. NOM can increase treatment chemical that is oxidant, coagulant, disinfectant demand and contribute to membrane fouling.

NOM can give drinking water objectionable colour, taste and odour. As a result, one goal of the drinking water treatment process is to reduce NOM concentration.

Measures to minimise water quality problem

The best practices for managing water quality problems resulting from organic matter are:

• to avoid excessive fertilization that can lead to an overabundance of phytoplankton,
• apply high quality feeds according to good feed management to minimize uneaten feed and assure a good feed conversion ratio,
• apply liming materials to ponds with acidic waters and bottom soils,
• avoid low dissolved oxygen concentration by limiting the intensity of culture or by providing adequate mechanical aeration.

Decomposition of ions in water

Decomposition of ions in water mainly occurs by the process of electrolysis.

Electrolysis

Chemical decomposition produced by passing an electric current through a liquid or solution containing ions.

Ions do not themselves decompose in water but ionic compounds decompose in water by the process of electrolysis.

Process of electrolysis

The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit. The desired products of electrolysis are often in a different physical state from the electrolyte and can be removed by some physical processes. For example, in the electrolysis of brine to produce hydrogen and chlorine, gaseous products are obtained. These gaseous products are collected as bubbles.

2 NaCl + 2 H2O → 2 NaOH + H2 + Cl2

A liquid containing electrolyte is produced by:

1. Solvation or reaction of an ionic compound with a solvent (such as water) to produce mobile ions
2. An ionic compound is melted by heating
3. An electrical potential is applied across a pair of electrodes immersed in the electrolyte.

Each electrode attracts ions that have opposite charge. Positively charged ions (cation) move towards the electron-providing (negative) cathode. Negatively charged ions (anion) move towards the electron-extracting (positive) anode.

In this process electrons are either absorbed or released. Neutral atoms gain or lose electrons and become charged ions that then pass into the electrolyte. The formation of uncharged atoms from ions is called discharging. When an ion gains or loses enough electrons to become uncharged (neutral) atoms, the newly formed atoms separate from the electrolyte. Positive metal ions like Cu2+deposit onto the cathode in a layer. The terms for this are electroplating, electrowinning, and electrorefining. When an ion gains or loses electrons without becoming neutral, its electronic charge is altered in the process. In chemistry, the loss of electrons is called oxidation, while electron gain is called reduction.

Decomposition potential

Decomposition potential or decomposition voltage refers to the minimum voltage (difference in electrode potential) between anode and cathode of an electrolytic cell that is needed for electrolysis to occur.

The voltage at which electrolysis is thermodynamically preferred is the difference of the electrode potentials as calculated using the Nernst equation. Often the actual voltage needed for electrolysis exceeds the thermodynamical value. The additional voltage is referred to as over potential and is especially high in electrolysis reactions that involve gases, such as oxygen, hydrogen or chlorine. Increasing voltage above the decomposition potential can increase the rate of reaction.

Oxidation and reduction at the electrodes

Oxidation of ions or neutral molecules occurs at the anode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:

Fe2+ (aq) → Fe3+ (aq) + e−

Reduction of ions or neutral molecules occurs at the cathode.

It is possible to reduce ferricyanide ions to ferrocyanide ions at the cathode:

Fe(CN)63- + e- → Fe(CN)64-

Neutral molecules can also react at either of the electrodes. For example: p-Benzoquinone can be reduced to hydroquinone at the cathode:

In the last example, H+ ions (hydrogen ions) also take part in the reaction and are provided by the acid in the solution, or by the solvent itself (water, methanol, etc.). Electrolysis reactions involving H+ ions are fairly common in acidic solutions. In aqueous alkaline solutions, reactions involving OH- (hydroxide ions) are common.

Sometimes the solvents themselves (usually water) are oxidized or reduced at the electrodes. It is even possible to have electrolysis involving gases.

Energy changes during electrolysis

The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (in theory) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy change divided by the free energy change of the reaction. In most cases, the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance, in the electrolysis of steam into hydrogen and oxygen at high temperature, the opposite is true and heat energy is absorbed. This heat is absorbed from the surroundings, and the heating value of the produced hydrogen is higher than the electric input.

Conclusion

In the past decades, natural organic matter (NOM), were commonly present in all surface, ground and soil water, has had an adverse effect on drinking water treatment. The existence of NOM results in many problems in drinking water treatment processes, and the properties and quantity of NOM can significantly affect the efficiency of these processes. NOM not only influences the water quality with respect to taste, colour and odour problems, but it also reacts with disinfectants, increasing the amount of disinfection by-products

Not only have that in the last few decades the quality of natural water has often worsened owing to contamination with artificial traced organic chemicals. These are sometimes carcinogens, mutagens or endocrine disruptors. Such substances are often not removed in traditional wastewater treatment and they are not easily biodegradable, also it may accumulate in organisms. Polluted water is a threat to both human and environments health.