COD or Chemical Oxygen Demand is the total measurement of all chemicals in the water that can be oxidized.
TOC or Total Organic Carbon is the measurement of organic carbons.
BOD or Biochemical Oxygen Demand is supposed to measure the amount of food (or organic carbons) that bacteria can oxidize.
TDS or Total Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (principally calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates) and some small amounts of organic matter that are dissolved in water.
Now in all these Oxidising processes we are eating available oxygen in natural water bodies like rivers, ponds and sea to keep the natural ecological life cycle going. Slowly it reduces to a level where baterias which reduce the organic matter to gas, water and soil die. that calls for death of small organisms, fish and algae. The water rotten up and becomes a dead water. Such water ponds, rivers start drying up to stop water from renewing. The rivers which are fed by rain or underground streams still keep running for a while as the dirt is pushed to larger rivers or to the sea. A famous example in India of dying river is River MITHI.
There are about 40 million organic compounds known in the environment which cannot be defined individually very easily. Therefore, the so-called sum parameters are used. The most popular sum parameter in waste water analysis are the BOD (biochemical oxygen demand), COD (chemical oxygen demand), TOD or Total Oxygen Demand and TOC or Total Organic Carbon. The TOC reflects the organic pollution on the basis of a direct carbon determination. 
But why does the BOD, COD, TOC, TDS etc. rises in the natural water bodies?
That's because of you and me.
We use a lot of popular household cleaners which are dangerously toxic. The truth is many ingredients in the common household cleaning products we’ve grown to trust are harmful to our health. Most consumers aren’t aware of the hidden dangers these popular cleaning products present to our health. Nor are they aware how pervasive these health threats are.
The most common methods of exposure are through the skin and respiratory tract. Children are frequently in contact with the chemical residues housecleaning products leave behind, by crawling, lying and sitting on the freshly cleaned floor. Children, especially infants and toddlers, frequently put their fingers in their mouths and noses, increasing risks for exposure. When infants eat solid food, how common is it that the food is placed directly on a high chair tray that has just been wiped down with a household cleaner or dish detergent. Another factor is that, pound for pound, childrens’ exposure levels are higher than adults’ because, although the amount of chemicals in an exposure remains equal, children’s bodies are smaller so the concentration is stronger, essentially. Also, their immune systems are still developing. Thus, children are probably the highest risk population for chemical exposures through cleaning products. For many of these same reasons, pets may also be at risk. Other populations with a pronounced risk are breast cancer victims, the elderly, asthma and allergy sufferers and those with compromised immune systems.
You may be thinking that the diluted aspect of off-the-shelf cleaning products reduces or altogether eliminates the threat of getting sick from your floor polish, window cleaner or air freshener. However many of the toxins found in these products (and so many other cleaning products) are bioaccumulative, meaning the chemicals do not purge easily from the body and over time even mild exposures can add up to toxic levels. In fact, a medical study recently conducted in Iowa suggests a correlation between certain occupations and bladder cancer. One of those occupations was cleaning services. These products are used repeatedly and routinely in the home to maintain cleanliness, increasing the chances for bioaccumulation of chemicals in the body. 
Most natural waters contain small quantities of organic compounds. Aquatic microorganisms have evolved to use some of these compounds as food. Microorganisms living in oxygenated waters use dissolved oxygen to oxidatively degrade the organic compounds, releasing energy which is used for growth and reproduction. Populations of these microorganisms tend to increase in proportion to the amount of food available. This microbial metabolism creates an oxygen demand proportional to the amount of organic compounds useful as food. Fish and aquatic insects may die when oxygen is depleted by microbial metabolism.
Some known ingredients and their hazards 
Found in: Many fragranced household products, such as air fresheners, dish soap, even toilet paper. being propitiatory IP protected etc. you won’t find phthalates on a label. If you see the word “fragrance” on a label, there’s a good chance phthalates are present.
Health Risks: Phthalates are known endocrine disruptors. Men with higher phthalate compounds in their blood had correspondingly reduced sperm counts. Unlike the digestive system, the skin has no safeguards against toxins. Absorbed chemicals go straight to organs.
2. Perchloroethylene or “PERC”
Found in: Dry-cleaning solutions, spot removers, and carpet and upholstery cleaners.
Health Risks: Perc is a neurotoxin, EPA classifies perc as a “possible carcinogen” as well. People who live in residential buildings where dry cleaners are located have reported dizziness, loss of coordination and other symptoms.
Found in: Most liquid dishwashing detergents and hand soaps labeled “antibacterial.”
Health Risks: Triclosan is an aggressive antibacterial agent that can promote the growth of drug-resistant Pathogens. Studies have now found dangerous concentrations of triclosan in rivers and streams, where it is toxic to algae.
4. Quarternary Ammonium Compounds, or “QUATS”
Found in: Fabric softener liquids and sheets, most household cleaners labelled “antibacterial.”
Health Risks: Quats are another type of antimicrobial, and thus pose the same problem as triclosan by helping breed antibiotic-resistant pathogens. They’re also a skin irritant.
Found in: Window, kitchen and multi-purpose cleaners.
Health Risks: 2-butoxyethanol is the key ingredient in many window cleaners and gives them their characteristic sweet smell. In addition to causing sore throats when inhaled, at high levels glycol ethers can also contribute to narcosis, pulmonary edema, and severe liver and kidney damage.
Found in: Polishing agents for bathroom fixtures, sinks and jewellery; also in glass cleaner.
Health Risks: Because ammonia evaporates and doesn’t leave streaks. That sparkle has a price. “Ammonia is a powerful irritant,” People who get a lot of ammonia exposure, like housekeepers, will often develop chronic bronchitis and asthma.” Ammonia can also create a poisonous gas if it’s mixed with bleach.
Found in: Scouring powders, toilet bowl cleaners, mildew removers, laundry whiteners, household tap water.
Health Risks: It’s a respiratory irritant at an acute level. It may be a serious thyroid disrupter.
8. Sodium Hydroxide
Found in: Oven cleaners and drain openers.
Health Risks: Sodium hydroxide is extremely corrosive: If it touches your skin or gets in your eyes, it can cause severe burns. Inhaling sodium hydroxide can cause a sore throat that lasts for days.
· A simple suggestion will stop all this for us. We need not connect our waste to common sewer line to send it to rivers or natural water bodies. We can digest it ourselves.
· Use Enzyme based cleaners and bacterial (good bacteria which associates with pathogens to curb them from harming human bodies) solutions.
· Reduce water load on sewer by using waterless systems and designs in sanitation space.