While designing the digestion tanks, the required capacity of the sludge digestion units depends on the following:
(i) The quantity of the sludge to be digested.
(ii) The quantity of the sludge remained after digestion.
(iii) The detention time or the period required for the digestion.
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The quantity of the raw sludge produced in the various sewage treatment units are given in table 18.1. As during digestion the organic solids get liquefied, gasified and mineralized, therefore the volume of the digested sludge becomes about one third the volume of the sludge. Thus the storage capacity of the sludge digestion units varies from 5/8th to 2/ 3rd the original volume of the sludge.
Mathematically, the required capacity of the digestion tanks
= (Period of digestion in days) × (5/8 to 2/3 the volume of incoming raw sewage daily)
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But as the digestion tanks destroy the volatile organic matters from the sludge therefore, their capacities are expressed in terms of the volatile organic material destroyed per day (or per month) per cu.m. of the tank volume. In most of the towns, the sewage sludge per capita is more or less constant; therefore the capacities of the sludge digestion units can also be expressed in terms of cu.m./capita/day.
Table 18.2 gives the approximate design capacities of digestion tanks.
Design of Digester Elements:
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Number of Units:
Conventional digesters are designed as single units for plants treating upto 4 mid. For larger plants, units should be provided in multiples of two, the individual capacity not exceeding 3 ml.d. High rate digesters are designed Comprising primary and secondary digestion tanks, each unit usually capable of handling sludge from treatment plants up to 20 mld.
Shape and Size of Tank:
Circular tanks are most common for sludge digestion and they should preferably be to less than 6 m not more than 55 m in diameter.
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Water Depth:
The side water depth of a digester may be kept between 4.5 to 6.0 m but in no case it should exceed 9 m. Depth of sludge in a digester has to be carefully worked out, because too deep digester causes excessive foaming and may chokage of the gas mains, building up high pressures in the digester causing a hazard. When the gas production reaches a figure of about 9 m3/d/m2 of top, surface of sludge, foaming become noticeable.
Therefore, before deciding the tank depth and surface area, the maximum gas production rate should be worked out. About 0.9 m2 gas is produced per kg of volatile matter destroyed. The optimum diameter or depth of the digester is calculated such that at twice the average rate of daily gas production, the value of 9m3/m2 of tank area is not exceeded.
Free Board:
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For fixed dome or conical roofs, free board between the liquid level and the top of the digester wall should not be less than 40 cm. For floating covers the free board between the water level and the top of the tank wall should preferably be not less than 60 cm. For fixed roof slab, free board should be 80 cm.
Bottom Slab/Characteristics:
The bottom of the digester should be given at least 1 in 12 slopes to facilitate easy withdrawal of sludge. The floor of the tank should be designed for uplift pressure due to the subsoil water or suitably protected either by anchoring or by pressure release valves.
Roof:
Fixed or floating roof can be provided in the sludge digester. R.C.C. domes, conical or flat slabs are used for fixed roof and steel domes are used for floating cover. Steel floating covers may either rest on the liquid or act as gas holders in the digesters themselves. When the floating cover is used for gas holder in a digestion tank, an effective vertical travel of 1.20 to 2.0 m should be provided.
The gas domes are provided in the roof at or near the centre of circular tanks. A gas take off point in the fixed or floating roof should be provided at least 1.0 m above the maximum liquid level in the tank. At least two manholes should be provided in the roof of the digester each of at least 75 cm dia.
For tanks more than 25 m dia at least four manholes should be provided for removal of excessive scum blanket and other matter which cannot be removed through sludge draw off pipes. During maintenance these manholes also serve to give adequate ventilation of the tank.
Pressure and vacuum release valves and flame traps should be provided in the roof covers. At least two access manholes with gastight and watertight covers are also to be provided in the operating gallery near the bottom of the side wall of the digesters. While the primary digesters are always constructed with covers with gas collection arrangements, the secondary digesters are often kept open.
This practice may not be objectionable since the maximum gas production and digestion take place in the primary stage itself. If the area has heavy rainfall, both the primary and secondary digesters should be covered.
Digester Control Room:
Normally a control room is provided near the digesters to house the piping and the process control equipment, which are mainly the sludge heating units, sludge transfer and recirculation pumps, sludge sampling sinks, blowers, thermometers etc. When the heating of sludge digester is done, the operations could be managed by locating conveniently, the necessary valves for supernatant and sludge withdrawal, in the digester wall itself.
Mixing of Digester Contents:
Mixing of the digester contents is an important requirement in sludge digestion. Mixing of digester brings about intimate contact between the raw sludge fed and the digesting sludge and maintain a rapid and uniform rate of digesting in the tanks. It also minimises scum formation, temperature stratification and dead zones in the digester.
In the conventional digesters, mechanical stirrers with revolving arms dipped a little below the scum level, equipped with vertical fingers or pickets extending downwards in the scum stratum, is provided which breaks the scum.
Another practice of compressing the sludge gas, and discharging at three more points in the supernatant zone below the scum zone is also adopted for minimising the scum. Scum can also be reduced by spraying the tank liquor or water when the scum is brought into the active digestion zone.
Gas Collection:
Sludge gas is normally composed of about 60-70% methane and 25- 30% carbon-dioxide by volume and small quantity of other gases such as H2S. The main combustion constitute in the gas is methane. Depending upon the sulphate content of the sewage and the sludge, the concentration of hydrogen sulphide in the gas varies.
H2S in addition to its corrosive properties imposes a limit on the utilisability or causes nuisance during the burning of the gas. Sludge gas having 70% methane has a fuel value of about 5800 kcal/m3. The average gas production is about 0.9 m3/kg of volatile solids destroyed at a normal operating pressure of 150-200 mm of water.
When the batch feeding is done in the digestion tank, the minimum and maximum gas production rates may vary from 45% to 200%, In case of continuous feeding system, the difference between the maximum and the minimum is considerably reduced. It is recommended to feed the high rate digesters with raw sludge and run the mixing device as continuously as possible to obtain not only a uniform rate of digestion but also uniform production of gas.
Care should be taken to collect the gas under positive pressure to prevent its mixing with air and causing explosion. The explosive range of sludge gas is between 5-15% by volume of gas with air. The gas dome above the digester roof should be used for taking out gas.
The gas holders should be designed to hold the quantity of gas as about 0.015 m3 to 0.019 m3 per capita per day. The velocity in sludge gas piping should not exceed 3.5 mps to prevent carryover of condensate from the condensate traps and avoid high pressure loss and damage to meters or flame traps and other appurtenances of the digestion system.