W2B-036 Sewage treatment-0.txt
SEWAGE TREATMENT

At the outset it is important to clarify two terms, sewerage and sewage, because they are often used incorrectly. Sewerage is a system of pipes used to collect and carry sewage which is the wastewater discharged from domestic premises. Domestic sewage consists of human wastes, paper, vegetable matter. This type of waste is organic because it consists of compounds of carbon and can be broken down by microorganisms into simpler compounds which are stable and not liable to cause a nuisance. Sewage can consists of 99% water and 0.1% solids.

Besides domestic sewage there is industrial waste. Many industrial wastes are also organic in composition and can be treated by microorganisms in the same way as domestic sewage. This type of treatment is called biological treatment and the strength of the sewage is measured in terms of B.O.D. or biochemical oxygen demand. This is a measure of the amount of oxygen used by the microorganisms in breaking down the sewage into stable compounds.

Thus in a town the sewerage system will collect the sewage from domestic, commercial, and industrial premises and carry it to the nearest river or to the sea. The dilutions available in the receiving water has traditionally determined the extent of treatment necessary.

SEWAGE TREATMENT PROCESSES

In modern treatment works sewage goes through several stages of treatment. Preliminary treatment to deal with large solids which are removed by screening or ore macerated and returned to the sewage flow. At this stage grit is removed in special tanks. The sewage then receives primary treatment which consists in allowing the sewage into sedimentation tanks where solids settle out in the form of sludge. The next stage is secondary treatment or biological treatment in which microorganisms are used to take organic matter out of solution so as to form a sludge which can be settled out in a final settling tank. Tertiary treatment can reduce the organic matter content still further if this is necessary. Nutrient removal consists of reducing the phosphorus and nitrogen in the sewage so as to prevent plant growth in the receiving waters.

HISTORICAL DEVELOPMENT OF SEWAGE TREATMENT

Early attempts at treating sewage in the U.K. consisted in acquiring large farms and spreading the sewage over the land where it decayed under the action of microorganisms. It was soon found that the land became'sick' and that it was necessary to treat the sewage. At first this was done by settling the sewage in concrete tanks in which the solid sewage settled out and was removed for further treatment. The partially clarified liquid sewage was then passed on to the land. Subsequently, chemicals such as lime were added to the settling tanks to increase the amount of solids being settled out.

A Royal Commission report of 1912 set a standard for the quality of treated sewage effluent before it could be discharged to rivers. The standard set was that the BOD should not exceed 20mg/1 and that the suspended solids should not exceed 30mg/1. These figures were recommended as a general standard and were related to situations where the receiving waters would dilute the treated effluent 8 times. The standards were designed so that the BOD in the receiving waters would not exceed 4 mg/1 and thus fish could survive. The limitation on suspended solids was to ensure that banks of sludge would not build up on the bed of the receiving waters.

BIOLOGICAL FILTERS

The sedimentation treatment described above even when aided by chemical treatment could not reach these standards. Thus a second stage or secondary treatment was required. At first secondary treatment consisted of treating the sewage on percolating filters. These consisted of circular beds of stone about 1m deep over which the settled sewage was distributed by sets of rotating pipes which sprinkled the sewage over the stones. Gradually a slime build up on the surface of the stones which absorbed dissolved material out of the settled sewage through microbial action and reduced the BOD of the effluent to 20mg/1. As the slime built up on the stone it sheared off particularly in the spring. It was necessary to trap this slime by passing the effluent from the filters through a second set of settling tanks. The effluent after this secondary treatment and settlement would in general meet the required standard of 20 mg/1 for BOD and 30 mg/1 for suspended solids.

ACTIVATED SLUDGE

In 1914 a different type of secondary treatment was developed. It was found by that continually aerating the effluent from the settlement tanks flocs of sludge would be formed and that they would absorb the dissolved organic matter out of the sewage in the same way as happens in the percolating filter. Over the years different forms of aeration developed. The retention period in these tanks was generally 8 hours and this became the conventional period for activated sludge systems. The sludge from conventional activated sludge was unstable and required further treatment by digestion which will be described later.

Quite a number of plants with percolating filters were build in towns throughout Ireland, one of the earliest being built in Tipperary town. However, the activated sludge plants were constructed because the system needed more precise control and was more suitable for a large population.

EXTENDED AERATION

In 1960 a system activated sludge based on a long period of aeration 24-36 hours was developed in Holland. Aeration was carried out in a continuous ditch known as an Oxidation Ditch. The method of aeration is by a horizontal shaft to which are attached a series of short paddles which dip into the sewage in the ditch. On rotating the shaft the sewage is aerated and propelled around the ditch. The aerated sewage is formed into a brown floc like sludge which settles out in a separate settling tank. Thus clear treated effluent is drawn off the bottom of the tank. The advantage of this system is that the sludge is stable and needs no further treatment except dewatering. The disadvantage is that the power absorbed is high because of the long period of aerationg, thus the system is suitable for small plants of less than say 20,000 population.

Subsequently a similar system of extended aeration using square tanks and vertical shaft aerations have been developed. These take up less space than the oxidation ditch but being deeper may be disadvantageous in bad ground conditions.

Since 1970 treatment plants in Ireland have been based on the extended aeration system whereas prior to that they were generally based on sedimentation and biological filtration. Treatment stages typical plants are shown in figs. 1 and 2.

It will be noted that preliminary treatment consisting of screening and grit removal are included in both systems but that in the modern plant these are mechanised.

When plants serve populations greater than 20,000 population it is usual to reduce power costs by providing sedimentation (primary treatment) and biological (secondary treatment).

TERTIARY TREATMENT
If the flow in the receiving water is less than eight times the quantity of effluent then a third stage of treatment is necessary. This is known as tertiary treatment and is aimed at reducing the BOD to 10 mg/1 and the suspended solids to 10 mg/1. Several forms of tertiary treatment are available for example:
(1) Passing the effluent over grass plots (2) Retaining the effluent in lagoons (3) Filtering through sand beds (4) Using a bed of pebbles or wedge wire in the final settling tanks.

There are a few small plants in Ireland which have tertiary treatment (using the grass plot or pebble bed / wedge wire system).

NUTRIENT REMOVAL

In addition to reducing the BOD and suspended solids consideration must nowadays be given to reducing the amount of phosphorus and nitrogen in effluents. These elements act as nutrients or fertilizers and when the effluent is discharged into a lake the phosphorus discharged with the effluent. This is done by adding aluminium or ferric salts to the treatment process. This has the effect of causing the phosphorus to settle out of the effluent and into the sludge. There are a few plants in Ireland where phosphorus removal is used, notably in Killarney, Mullingar and Castleblaney.

Where the effluent is discharged into the sea it may be necessary to reduce the nitrate content by a process of denitrification which will reduce the nitrate content to 10 mg/1. So far it hasn't been necessary to use denitrification in Ireland.

SLUDGE TREATMENT

The sludge from extended aeration plants is rendered stable by the treatment process and can be dewatered by passing it through a filter belt press thus thickening the sludge from 1% solids to 15 or 20% solids. The sludge can then be mixed with domestic refuse on a tiphead. The sludge from primary settling tanks is unstable and needs further treatment by digestion in closed tanks. This is known as anaerobic sludge digestion and can be carried out in unheated tanks or in heated tanks. When the tanks are heated the methane gas which is formed can in turn be used to fuel burners which keeps the tanks heated.

Another form of sludge treatment is composting. This consists of mixing the sludge with straw of lime and cement kiln dust. The sludge will heat up and kill of microorganisms. This system has been used in the USA and on the continent and and has recently been introduced into the North of Ireland.
The final disposal of sewage sludge can be by a number of routes
(1) Disposal on agricultural land (2) Co-disposal with domestic refuse on a landfill site (3) Disposal in the sea (4) Incineration

The disposal of sludge to agricultural land is controlled by EC directive No. 86/278/EEC. This requires that the sludge, unless it is injected or ploughed immediately into the soil, must undergo biological, chemical or heat treatment, long term storage on any other appropriate process.

The object of this treatment is to kill off disease causing organisms which may be in the sludge. These restrictions are difficult to meet in the case of small plants and the amount of sludge used in agriculture has declined in favour of co-disposal with domestic refuse in landfill sites.

Disposal of sludge at sea is only practised in the case of Dublin and it has been decided to phase this out.

Incineration is only suitable for large populations and the system is not used in Ireland.

Earlier incinerators proved very expensive to operate and for this reason many of the plants built in other countries have been abandoned. In more recent years a new form of incinerator has been developed based on the use of a fluidised bed which it proving more successful.

SEWERAGE AND SEWAGE TREATMENT FACILITIES IN IRELAND

At present approximately 66% of the population live in towns and are connected to sewers. The remainder live in rural areas, and it is impractical to connect them to sewers. These houses are served by septic tanks.

The number and capacity of the sewage treatment plants in the Republic of Ireland are shown in Table 1.

Most Irishtowns are located around the coast, and by far the largest of these is Dublin, with 920,000 inhabitants. The sewage from 600,000 of these receives primary treatment and there are two sea outfalls which serve 260,000 and 60,000 people.

47 other towns around the coast discharge untreated sewage to estuaries or coastal waters, and only 11 coastal towns have primary or secondary treatment. There are 68 towns which discharge to inland waters. These are towns between 1,000 and 20,000 inhabitants and the majority of these have now full secondary treatment.

The Environment action programme announced by the Minister for the Environment in January 1990, provides for the elimination of untreated discharges from major coastal towns at an estimated cost of £ 400 million and from inland towns at an estimated cost of £ 230 million by the year 2000.
