ACTIAVTED SLUDGE WASTE WATER TREATMENT AT UGANDA CHRISTIAN UNIVERSITY

1.1  TREATMENT AT THE ACTIVATED SLUDGE WASTEWATER TREATMENT  PLANT

This involves removal or reduction of environmentally unfriendly components from wastewater. The choice of treatment depends largely on the type of wastewater. Wastewater is divided into two major types, industrial wastewater and domestic wastewater. Industrial wastewater is a product of industrial works, while domestic wastewater is that which comes from man’s daily chores. At UCU wastewater generated is domestic since it is generated by daily chores such as bathing, doing laundry, flushing toilets, preparing meals, washing dishes among other activities. Industrial wastewater has a greater percentage of environmentally toxic substances compared to domestic wastewater, however; according to NEMA, domestic wastewater must be properly treated because it contains excessive nutrients, harmful bacteria/viruses and household chemicals that may contaminate the land and waters of the environment and threaten public health. Domestic wastewater treatment at UCU is done by an activated sludge wastewater treatment plant.

1.2  COMPONENTS OF AN ACTIVATED SLUDGE WASTEWATER TREATMENT PLANT

1.2.1        Intake works

Wastewater that enters the plant is called influent. It enters the plant by gravity through the intake works. The intake works is a channel that consists of screens that helps trap large inorganic materials which are untreatable and also further breakdown very large organic material for example faeces.

1.2.2        Parshial flume

Parshial flume is used to measure the flow rate of influent into the plant. The parshial flume has screens at its outlet through which wastewater enters the equalization chamber. The screens help to trap inorganic matter that was not trapped via the intake works.

1.2.3        Equalization chambers

These are two chambers in which the level of sewerage is equalized. The level of wastewater in both chambers is balanced and in the process it helps to equalize the pH throughout the wastewater hence neutralizing it. The chambers also have pumps which help pump wastewater into the V-notch weir.

1.2.4        V-notch weir

Wastewater must spend between 6-24 hours under aeration. To attain this, the flow rate of influent into the aeration chamber is controlled from the v-notch weir.

1.2.5        Aeration chamber

This is the chamber in which oxidation of wastewater takes place. Air is supplied to this chamber by blowers powered by electric motors of 14 horse power. Air from the blowers flows into vertical down feed pipes found along the longer side of the chamber, then into lateral horizontal pipes found at the bottom of the aeration tank, and finally into air diffusers. When wastewater enters this chamber, it is mixed with aerobic bacteria so that the bacteria get into contact with the organic material to feed on, and it is also aerated since the aerobic bacteria need air or dissolved oxygen to survive. The mixing and aeration is done by diffused aerators. Aerobic bacteria multiply by consuming the organic material in the wastewater. Breakdown of the organic material by bacteria is done both by adsorption and absorption, adsorption involves attachment of food to the outer slime layer that surrounds the bacteria while absorption involves taking in of food through the cell membrane of the bacteria. It was important to note that for efficient functioning of the system, amount of food supplied to bacteria should not be too much or too little. Specific amount of bacteria can consume certain amount of food in a specific duration of time. Per day, 450g of bacteria can consume about 20-70g of organic material. The wastewater treatment plant at UCU was designed to treat 160,000 litres of wastewater per day, this semester it was receiving 150,000 litres per day.

1.2.6        Clarifier 

This is the chamber where the effluent is allowed to settle so that it separates from the microorganisms. Effluent is the water product after treatment in the aeration chamber. The separated effluent leaves the clarifier through overflow weirs into effluent troughs. The weirs are laid in the clarifier such that their outlets are at the top of the effluent level. This is so that all the effluent that gets into the chlorination chamber is free of solid material such as sludge. The floating solids are removed by a skimmer.

1.2.7        Chlorination chamber

This is the chamber in which chlorination of effluent is done. Chlorination is done at a controlled rate with the aid of a gravity dozer. The recommended rate of chlorination is 0.25mg/l to completely kill pathogens. As effluent leaves this chamber, it passes through the roughing filters which help to remove solid material like leaves before it is released to the environment. At UCU, since the effluent is usually too much with nowhere to be disposed off, eucalyptus trees were grown near the treatment plant so that they can suck up majority of the water released from the chlorination chamber onto the land. One eucalyptus tree consumes 250 litres of water.

1.3  SLUDGE

Sludge is the solid that remains after wastewater treatment. This sludge has bacteria which have become hungry due to less organic material found in the effluent in the clarifier. At the clarifier, where sludge is allowed to settle at the bottom of the effluent, the return sludge airlift pump returns sludge to the aeration chamber where it is again used to breakdown organic material in the influent received by the aeration chamber.

1.3.1        Sludge wasting

After a while, the sludge becomes too much for the system, this can cause the system to choke with solids so it needs to be removed by a process called sludge wasting. This is done by diverting some of the sludge that is being returned to the aeration chamber from the clarifier. At the UCU treatment plant, when sludge wasted it is pumped to a sludge holding tank for 15 minutes. The sludge holding tank had diffusers that operate at the same time with the blowers of the plant, so that the sludge is kept aerobic for disposal. In case of too much wasted sludge, the supernatant which is the overflow sludge is returned to the aeration chamber The thickened sludge in the tank is then pumped to the sludge lagoon where it is allowed to dry up forming a sludge cake.

1.4  TESTS CARRIED OUT

1.4.1        Settleability test

This is a test that measures how well the sludge in the wastewater settles. It is carried out at the same time and at the same area of the aeration chamber away from any return sludge pipe or influent pipe. Using a basin, a sample was taken from the aeration chamber at 10:10am after the blower had operated for 10 minutes. The sample was then placed in a 1000 ml graduated cylinder where it was allowed to settle for 30 minutes, however it is observed for the first 5 minutes to see how the sludge settles. The sample compacted very fast leaving a dark brown cloudy supernatant indicating old sludge thus a need for sludge wasting . After the 30 minutes, the sludge had settled at 60% of its original volume indicating a fairly good sludge settling rate.

1.4.2        50% Dilution test

Regarding the 60% settling after the 30 minutes, a dilution tests was carried out to determine whether there was too much or too little sludge in the system. 50% of the original sample was removed and the remainder mixed with distilled water. The mixture was then allowed to settle for 30minutes, after which the sludge settled better than the original sample. This indicated old sludge in the system hence a need to sludge waste.

1.5  NTAWO BOOSTER STATION

1.5.1        Pump house

A visit was made to the wetland at Ntawo which is the main source of water to UCU. The pump house built by the water body uses lift pumps to pump water from the well to a water tank. From the water tank, the water flows at a controlled rate, into a horizontal flow sedimentation tank.

1.5.2        Horizontal flow sedimentation tank

Water is sent to the sedimentation tank so that it is allowed clear i.e. the particles in the water are allowed to settle. 45 to 60 milligrams of Aluminium sulphate per litre of water is added to increase the rate of settling by forcing small particles to come together by a process known as coagulation. Because of the solids that settle at the bottom, the tank at Ntawo which is cleaned every after three weeks was cleaned during the visit. This was done by first turning of the lift pumps to stop water from flowing, then opening the outlet valves of the tank to allow water in the tank to flow back into the well. When all the water was out, the inner wall and floor of the tank were cleaned using scrubbing brushes and water.

>picture of cleaning<

After the cleaning was done, the outlet valves of the tank were closed, the lift pumps were turned on and the water flowing into the sedimentation tank was pre-chlorinated using chlorine tablets. These were put in a chlorine tank which had a tap through which the dissolved chlorine was administered to water entering the sedimentation tank at a controlled rate.

1.5.3        Filters

The water from the sedimentation tank flows into rapid sand filters. When water passes through the filter by applied pressure, the particles that didn’t settle in the sedimentation tank are trapped by the sand filter bed. The sand filters at Ntawo were two in number, one was operational and the extra one was a standby in case the operational filter needed cleaning. It was explained that for sand filters, loading rate is the rate at which water is supplied to the the sand filter. The maximum loading rate of any sand filter is 5 m³/m²h. This means that for a sand filter of area 1m² the rate at which water is let in should not exceed 5000 litres per hour. At Ntawo, the rapid sand filters had an area of 2.25 m². When calculated maximum loading rate obtained was 11,250 litres of water per hour. Using such data, the inlet velocity of the water entering the filter can be adjusted accordingly. It is from the filters that water is then pumped to the booster house at the university. The booster house then pumps the water to the main tanks that supply the university with water.

UCU - UGANDA CHRISTIAN UNIVERSITY