How Communities Used to Deal With the Disposal of Wastes? (Part 2)

How Communities Used to Deal With the Disposal of Wastes? (Part 2)

In the first article, we’ve discussed how ancient communities like the nomadic tribes used to dispose of an astonishing amount of waste materials. Today, we shall take a closer look at how a fully efficient sewage system was developed.

The Work of Sanitary Engineers

The Work of Sanitary Engineers

In the sewage-disposal plant, sanitary engineers were ready for the task of rendering sewage harmless or even useful. One of the first measures they adopted was that of screening. As the screening entered the treatment plant, it passed through a set of screens that removed coarse suspended matter such as rags, sticks and floating orange peels. In the smaller plants, the operators raked this material off the screen and hauled it away to be buried.

Larger plants often use grinding machines that automatically rake the material off the screen, grind it and then return it to the sewage flow. Some plants dispense with screens entirely – the sewage passes through a grinding machine called a comminutor, which shreds the coarse material as it flows by.

The next step is to remove heavy inorganic matter such as sand. The sewage is directed through a compartment known as a grit chamber. This is so designed as to allow the sewage to flow through it at some established rate such as one foot per second, regardless of the amount of sewage entering the chamber. At this rate of flow, the inorganic, heavy sand and similar particles drop out and the lighter organic material flows on for further treatment.

Next, the sewage flows to settling tanks where that portion of the polluting material that is in suspension is given an opportunity to settle out. The settling tanks in a sewage-treatment plant are very much like those used in a water-supply system. They hold the sewage for some specified length of time, say two or three hours –in the course of this time a substantial portion of the sewage material settles to the bottom of the tanks as sludge.

The three preliminary methods that I have just described made up what sanitary engineers call “primary treatment.” They will generally remove 60 percent of the sewage’s suspended solids and 35 percent of its biological-oxygen demand. In many cases, when the sewage is not strong and the stream into which it empties is large and relatively unpolluted, this degree of treatment is considered adequate.

Purification by the primary treatment process can be increased if necessary by the use of chemical coagulants like those used in treating drinking water. The operator adds aluminum sulfate or iron salts to the sewage and produces a floc – a light and loose mass that will settle and take with it a large part of the offending sewage material. Treatment in this manner results in removing from 80 to 90 percent of the suspended solids and from 65 to 70 percent of the biochemical-oxygen demand. A plant that uses chemical coagulants is located near the famous Coney Island bathing beach in New York City. In the spring and summer, chemical coagulants are added to purify sewage more thoroughly and thus to safeguard the health of swimmers at the beach. In the fall and winter, when there is no swimming, no chemicals are used for purification.

In many cases, primary treatment of sewage does not suffice. Even with 85 percent of the suspended solids removed, the effluent or outflow will not be free enough from pollution to avoid trouble if the receiving stream is small or if the sewage is excessively strong. Consequently, the sanitary engineers reduce the organic material still remaining in the sewage by a biological process- that is, by causing living organisms to attack it. They begin this process by introducing air into the sewage – this encourages the growth of bacteria, which attack the organic matter in the wastes and decompose it quickly.

Stay tuned for a part three!


How Communities Used to Deal With the Disposal of Waste?

How Communities Used to Deal With the Disposal of Waste?

In any community, large or small, an astonishing amount of waste materials accumulates even in the course of a single day. There are human wastes resulting from the chemical changes that food and drink undergo in the body. There are large quantities of household wastes. A housewife cuts away certain portions of vegetables and other foods in preparing a meal and after that meal, bones, fruit rinds and uneaten food must be disposed of.

Old shoes, boxes, crates, cartons, useless articles of metal and rubber, discarded toys and whatnot add to the pile of refuse. There are industrial wastes too resulting from a vast variety of manufacturing operations. All of these waste materials must be disposed of in some way or other.

In the average fair-sized community, certain wastes are carried away by water through a sewage system. Others, such as garbage, trash, ashes and brushwood, are collected periodically and carried to a central disposal plant.

So, lets’ discover how previous communities dealt with this age-old problem.

The Disposal of Wastes

The Disposal of Wastes

The disposal of sewage, as we know it today, is a recent engineering practice.

There were sewers in antiquity in India, Rome and a few other places, but they served mainly to collect storm water or like Rome’s famous Cloaca Maxima to drain marshy areas.

If you go back to the middle ages, sewage flowed along open drains that ran through the streets. Later, it was carried in conduits to open cesspools located on the outskirts of cities. It was not until the nineteenth century that modern practices of sewage disposal were widely adopted. Even today, in many communities, particularly in the Far East, men come to homes at night, collect night soil (human wastes) and carry it away in carts.

In certain places, particularly China and Japan, night soil is used extensively as fertilizer – that which is collected from the wealthier sections of the community commands the highest prices. This practice has its good points, since it returns to the soil certain valuable organic materials that were taken from it when the crops were harvested. However, most of us dislike the idea of having human excretion hauled through the streets and health authorities do not like to have us eat raw foods grown in excretion-fertilized soil.

Our sanitary engineers use an entirely different method of disposing of human wastes and certain other types of refuse. They collect them in a system of sewers, bring them to a sewage-disposal plant, treat them so as to render them harmless and then empty the purified sewage into seas, lakes or rivers.

In the best systems, the sanitary sewers servicing homes are separate from the storm sewers that take the storm waters from streets. In the older systems, however, sanitary and storm sewage are carried in the same pipes. It is easy to understand why sanitary engineers are opposed to such combined sewers, as they are called.

After all, storm water is still rain water, except for a little debris that is washed off the streets, it is unpolluted and it is not necessary to treat it before it is emptied into a receiving stream. It is sheer waste to build sewage-treatment plants that will have to be big enough to treat not only sewage but also comparatively pure rain water that requires no particular treatment.

The nomadic tribes of old were not bothered at all by the problem of waste collection. They merely cast out their refuse and let the weather destroy it or the birds and beasts eat it. However, as communities grew larger, men came to recognize that they would have to adopt some method or other of collection and disposal.

Today, things have changed and more sustainable principles were implemented to ensure a more environmentally-friendly waste disposal. If you want to learn more about it, wait for the second article.