In this article we will discuss about:- 1. Types of Anaerobic Sludge Digesters 2. Capacity of Standard Rate or Low Rate Sludge Digesters 3. Anaerobic Digester Elements 4. Performance of Digesters.
Types of Anaerobic Sludge Digesters:
The anaerobic sludge digesters are of the following two types:
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(a) Standard rate or Low rate or Conventional sludge digesters
(b) High rate sludge digesters
(a) Standard Rate or Low Rate or Conventional Sludge Digesters:
Standard rate or low rate sludge digestion is the simplest and the oldest process. The basic features of this process are shown in Fig. 16.6 (a). Raw sewage is fed into the digester intermittently. Bubbles of sewage gas are generated and their rise to the surface provides some mixing, but otherwise the contents of the digester remain unmixed.
As a result the digester contents are allowed to stratify, thereby forming four distinct layers- a floating layer of scum, layer of supernatant, layer of actively digesting sludge, and a bottom layer of digested sludge. The decomposition is essentially restricted to the middle and the bottom layers. Stabilized sludge which accumulates and thickens at the bottom of the tank is periodically drawn off from the centre of the floor. Supernatant is removed from the side of the digester and recycled back to the treatment plant.
Fig. 16.6 Typical Anaerobic Sludge Digestion Processes (a) Low Rate Single Stage Process. (b) High Rate Stage Process. (c) Two Stage Process.
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The standard rate or low rate sludge digestion process is usually a single stage sludge digestion process in which sludge digestion, sludge thickening and supernatant formation takes place simultaneously.
The raw sludge is fed into the digester through a pipe which terminates near the centre of the tank. The digested sludge is withdrawn from the tank bottom and supernatant is removed from the side of the tank.
The gas produced during the process of digestion is collected in the dome shaped top portion of the tank and is taken out through a gas pipe. The contents of the digester in this case are usually unheated and unmixed.
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However, the contents of the digester are mechanically stirred with revolving arms dipping a little below the scum level equipped with vertical fingers or pickets extending downwards in the scum stratum which pass between fixed pickets, breaking up the scum.
Lack of proper mixing in the standard rate or low rate sludge digesters leads to stratification. As such in this type of digesters much of the digester volume is wasted and sometimes acidification takes place in the top and middle layers while methane formation is confined to the lower layers.
This can lead to areas of low and high pH in the tank which restrict optimum biological activity. Also the chemicals added for pH control are not dispersed throughout the tank and their effectiveness is limited. Grease breakdown is poor because the grease tends to float to the top of the digester, while the methanogenic bacteria are confined to the lower layers.
Further methanogenic bacteria are removed with the digested sludge and are not recycled to the top where they are required. During progression from top to bottom of the digestion tank the sludge is compressed and gradually dewatered. The required detention times for these digesters vary from 30 to 60 days.
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(b) High Rate Sludge Digesters:
The high rate sludge digestion process differs from the standard rate or low rate sludge digestion process in that the solids loading rate is much greater (see Table 16.4). The basic features of the high rate sludge digestion process are shown in Fig. 16.6 (b).
In this case raw or pre-thickened sludge is fed into the digester continuously at a more or less uniform rate and the contents of the digester are heated and completely mixed. Pre-thickening of raw sludge and heating of the digester contents are, however, optional.
The effect of these features on the sludge digestion process is indicated below:
(i) Complete mixing of sludge in high rate sludge digesters creates a homogeneous environment throughout the digester. It also quickly brings the raw sludge into contact with micro-organisms and evenly distributes toxic substances if any, present in the raw sludge. Furthermore, when stratification is prevented because of mixing, the entire digester is available for active decomposition, thereby, increasing the effective solids retention time.
(ii) Pre-thickening of raw sludge before digestion results in the following benefits:
1. Large reduction in digester volume requirements.
2. Less heating energy requirements.
3. Less mixing energy requirements.
4. The thickener supernatant is of far better quality than digester supernatant, thereby it has less adverse impact when returned to the sewage treatment stream.
There is, however, a point beyond which further thickening of raw sludge has following effects on digestion:
The higher solids concentration beyond 6% in the digester affects the viscosity which in turn affects mixing and hence deserves special consideration.
In case of highly thickened raw sludge the concentration of salts and heavy metals present in the raw sludge and end products of digestion such as volatile acids, ammonium salts may exceed the toxic levels.
(iii) Sludge temperature is one of the important environmental factors. Where the digester sludge temperatures are low, digester heating is beneficial because the rate of microbial growth and therefore, the rate of digestion increases with temperature.
In case of high rate sludge digesters because of good mixing, there is no stratification and hence no loss of capacity due to scum or supernatant layers. Further by adopting more or less continuous addition of raw sludge and resorting to pre-thickening of the raw sludge to a solids content of about 6%, the required detention time is only about 15 days or less, which is less than that for low rate sludge digesters.
The high rate sludge digestion process shown in Fig. 16.6 (b) is a single stage sludge digestion process in which the incoming sludge continuously displaces the digested sludge and hence there is no separation of the supernatant in the tank.
Further the total solids are reduced by 45 to 50% and given off as gas. Because of these factors the thickening of the digested sludge cannot take place and it is about half as concentrated as the untreated sludge feed.
On account of these limitations the high rate sludge digestion process is usually carried out as two stage sludge digestion process which is described below:
i. Two Stage Sludge Digestion Process:
The basic features of the two stage sludge digestion process are shown in Fig. 16.6 (c). In this process two digestion tanks are used which are operated in series. The first tank known as primary tank or primary digester or first stage digester is used for the digestion of the sludge. It is equipped with heating and mixing facilities.
The second tank known as secondary tank or secondary digester or second stage digester is used for storage and concentration of the digested sludge and for formation of a relatively clear supernatant. Frequently, the tanks are made identical, in which case either one may be used as the primary digester or first stage digester. In other cases, the second tank may be an open tank, an unheated tank, or a sludge lagoon.
ii. Digester Capacity:
The capacity of digesters depends on:
(i) Daily volume and moisture content of influent sludge and digested sludge;
(ii) Temperature of digestion;
(iii) Desired degree of destruction of volatile solids; and
(iv) Storage capacity for digested sludge.
(i) Daily Volume and Moisture Content of Sludge:
The volume of sludge produced daily varies depending on the degree or removal of suspended solids in primary and secondary settling tanks, moisture content and specific gravity of sludge. Further typical values of the solids that may be present in the sludges produced in various sewage treatment processes are indicated in Table 16.1.
(ii) Temperature of Digestion:
The process of sludge digestion is greatly influenced by temperature, the rate of digestion being more at higher temperatures and vice-versa. The effect of temperature on the digestion period is shown in Table 16.3 as well as in Fig. 16.8. Depending upon the temperature range, different kinds of micro-organisms are active in the digester.
For an operating temperature range of 20° to 40°C, the range is known as mesophilic and 40° to 60°C the range is known as thermophilic. The ambient temperature in our country is generally favourable for operation under mesophilic condition, throughout the year. However, where extremely low temperatures are likely to be encountered, it may be necessary to heat the digesters in specific periods of the year.
(iii) Desired Degree of Destruction of Volatile Solids:
The percentage of volatile solids destroyed is dependent on the percentage of volatile solids present in the raw sludge. The detention period given in Table 16.3 would accomplish about two-thirds destruction of volatile solids for a 70% volatile solids normally encountered in raw sludge. The different values of volatile solids destruction when digestion is considered satisfactory are shown in Fig. 16.9.
(iv) Storage Capacity for Digested Sludge:
Storage capacity for digested sludge is required in places where digested sludge is applied to drying beds for dewatering, and the use of sludge drying beds is interrupted during monsoon periods. This additional capacity requirement can be met either by increasing the digester capacity or by providing a separate digested sludge holding tank. Normally, an additional 10 to 15 days digested sludge storage capacity is sufficient. However, if local meteorological data is available, such data should be used to determine the capacity needed for storage.
Capacity of Standard Rate or Low Rate Sludge Digesters:
If the progress of the sludge digestion is assumed to be linear then, as shown in Fig. 16.10, the required capacity of the digester is given by the expression-
Additional capacity is required to store digested sludge during the monsoon period which is given by the expression-
Further additional storage capacity is also required to be provided to compensate for grit accumulation and free board as indicated below:
(i) Maximum Grit and Scum Accumulations:
Considerable amount of grit and scum may accumulate before a digester is cleaned. This reduces the active volume of the tank. Hence about 0.6 to 1.0 m additional depth for grit and scum accumulation must be provided.
(ii) Freed Board:
About 0.6 to 0.8 m free board (from rim of the digester wall to the highest liquid level) must be allowed for differences in the rate of feeding and withdrawing and to provide reasonable operational flexibility.
Capacity of High Rate Sludge Digesters:
The high rate sludge digesters are generally two stage sludge digesters which are provided with two tanks, the capacities of which may be determined by the following expressions-
In this case also further additional storage capacity to compensate for grit accumulation and free board is provided.
Check for Digester Capacity:
The digester capacity must be sufficient to prevent the process from failing under all accepted conditions. The process may fail due to accumulation of volatile acids resulting in decrease in pH and when volatile acids/alkalinity ratio becomes greater than 0.5 the cessation of methane production occurs.
Further the digester capacity must also be large enough to ensure that raw sludge is adequately stabilized for which it is necessary that the micro-organisms are given sufficient time to reproduce so that they can:
(a) Replace the cells lost with the withdrawn sludge and
(b) Adjust the microbial mass to the organic loading and its fluctuation.
The digester capacities determined by the above noted equations needs to be checked in respect of these conditions. The two key design parameters used for this purpose are volatile solids loading rate and Solids Retention Time (SRT).
The weight of Volatile Suspended Solids (VSS) applied per day per unit capacity of the digester represents the volatile solids loading rate and it is expressed as kg VSS/day/m3. Table 16.4 gives the typical volatile solids loading rates used for design purposes.
The Solids Retention Time (SRT) is the average time a unit of microbial mass is retained in the digester. In the anaerobic digesters without recycle, the solids retention time (SRT) is equivalent to the hydraulic retention time (HRT), i.e., volume of digester/volume of sludge withdrawn per day. Table 16.4 also gives typical values of solids retention time (SRT) and hydraulic retention time (HRT) used for design purposes.
It has been shown experimentally that percentage of destruction of volatile solids and formation of methane decreases as the solids retention time (SRT) is reduced. The solids retention time (SRT) can, however, be lowered to a critical value (SRTC.) beyond which the process will fail completely.
As such the size of the anaerobic digester should be adequate enough to ensure that the solids retention time in the digester is always well above the critical value (SRTC). Since temperature has an important effect on bacterial growth, the solids retention time (SRT) depends on temperature.
Typical values of solids retention time at different temperatures which may be adopted for the design of high rate sludge digesters are given in Table 16.5 in which the critical values (SRTC) are also given:
Anaerobic Digester Elements:
(a) Number of Units:
Standard rate or low rate sludge digesters are designed as single units for plants treating upto 4 Mld. For larger plants, units are provided in multiples of two, the capacity of the individual unit not exceeding 3 Mld.
High rate sludge digesters are designed as two stage sludge digesters comprising primary and secondary digestion tanks, each unit generally capable of handling sludge from treatment plants upto 20 Mld.
(b) Digester Shape and Size:
The digesters are mostly low vertical tanks of cylindrical shape with diameters ranging from 6 to 35 m and height ranging from 6 to 12 m. It has been found that digester mixing is effective when the ratio of digester diameter to sludge depth is between 1.5 to 4.
(c) Floor Slope:
The digesters are provided with hopper shaped bottom floor with slope in the range of 1 in 6 to 1 in 10 to facilitate easy withdrawal of digested sludge. The digester floor should be designed for uplift pressure due to the subsoil water or suitably protected by anchoring.
(d) Roofing:
Sludge digesters can have either fixed or floating roofs. Reinforced cement concrete domes, conical or flat slabs are used for fixed roof and steel domes are used for floating covers. Steel floating cover may either rest on the liquid or act as gas holders in the digesters themselves. If a floating cover is used for gas holder in a digestion tank, and effective vertical travel of 1.2 to 2 m should be provided.
(e) Water Depth and Free Board:
Side water depth of a digester may be kept between 4.5 to 6.0 m, and it should not exceed 9 m even for very large tanks.
The depth of the digester has to be worked out carefully. Too deep a digester causes excessive foaming which may result in choking of the gas pipes and building up high pressure in the digester. In the case of standard rate or low rate digesters when the gas production reaches a figure of about 9 m3 day/m2 of top surface of sludge, foaming becomes noticeable.
Therefore, before the tank depth and surface area of a digester are worked out, maximum gas production rate should be determined. An average of about 0.9 m3 of gas is produced per kg of volatile solids destroyed. The optimum diameter or depth of digester is calculated such that at the average rate of daily gas production, the value of 9 m3 per m2 of tank area is not exceeded.
The free board is dependent upon the type of cover and the maximum gas pressure. For fixed dome or conical roofs free board between the liquid level and the rim of the digester wall should not be less than 0.6 m. For flat covers, the free board between liquid level and the top of the tank wall should preferably be not below 0.6 m. For fixed slab roofs, a free board of 0.8 m is recommended.
(f) Mixing of Digester Contents:
A certain amount of natural mixing occurs in anaerobic digester caused by both the rise of sludge gas bubbles and the thermal convection currents created by the addition of heated sludge. This effect of natural mixing is significant, particularly in the case of high rate sludge digesters fed continuously. However, in the case of high rate sludge digesters only this natural mixing is not sufficient to ensure stable performance of the digestion process.
Therefore, artificial mixing of sludge is carried out in the digesters for which the following methods are used:
(1) External pumped circulation
(2) Internal mechanical mixing
(3) Internal gas mixing
External pumped circulation while relatively simple is limited in application because of large flow rates involved. This method can, however, achieve substantial mixing, provided that sufficient energy in the range of 5 to 8 watts/m3 is dissipated in the digester.
More energy will be required if piping losses are significant. Pumped circulation allows external heat exchanges to be used for heating the digester contents and uniform blending of raw sludge with heated circulating sludge prior to the raw sludge’s entry to the digester.
Internal mechanical mixing of digester contents may be accomplished by means of propellers, flat- bladed turbines or draft tube mixers. Mechanical mixers can be installed through the cover or walls of walls of the digester. Substantial mixing can be effected with about 5 to 8 watts/m3 of digester contents.
Internal gas mixing devices normally used for digesters are of the following types:
(i) The injection of a large sludge gas bubble at the bottom of a 300 mm diameter tube to create piston pumping action and periodic surface agitation.
(ii) The injection of sludge gas sequentially through a series of lances suspended from the digester cover to as great depth as possible depending on cover movement.
(iii) The free or unconfined release of gas from a ring of spargers mounted on the floor of the digester.
(iv) The confined release of gas within a draft tube positioned inside the tank.
(g) Gas Composition and Collection:
Sludge gas is normally composed of 60 to 70% methane and 25 to 35% carbon dioxide by volume and small quantities of other gases like hydrogen sulphide, hydrogen, nitrogen and oxygen. Total gas production is usually estimated from the volatile solids reduction or from the volatile solids loading of the digester.
Typical values of gas production are from 0.75 to 1.12 m3/kg of volatile solids destroyed, and from 0.5 to 0.75 m3/kg of volatile solids added. Gas production can also be approximately estimated on a per capita basis.
The normal yield of gas is about 0.015 to 0.022 m3 per capita per day in primary plants treating normal domestic sewage, and in secondary treatment plants, this is increased to about 0.028 m3 per capita per day. The average value of gas production is usually taken as 0.9 m3/kg of volatile solids destroyed, or 0.6 m3/kg of volatile solids added to the digester, or 0.025 m3 per capita per day.
The combustible constituent in the gas is primarily the methane. Sludge gas containing 70% methane has a fuel value of about 5800 k cal/m3 and may be used for heating digestion tanks and treatment plant buildings or in gas engines to drive pumps, generators, or air compressors.
The concentration of hydrogen sulphide in the sludge gas varies depending upon the sulphate content of the sewage and the sludge. Hydrogen sulphide in addition to its corrosive properties imposes a limit on the usability or causes nuisance during the burning of the gas.
Sludge gas is collected under positive pressure to prevent its mixing with air and causing explosion. The explosive range of sludge gas is between 5 to 15% by volume of gas with air. The gas may be collected directly from under a floating cover on the digester or from the fixed cover by maintaining a constant water level.
Where primary and secondary digesters are provided to operate in series with the primary digesters having a fixed cover and the secondary digesters with a gas holding or floating cover, the gas piping from each digester should be interconnected (see Fig. 16.6 c). A separate gas holder may be provided to collect the gas from the primary digester where the secondary digester is kept open.
A gas dome above the digester roof should be used for gas take off. The velocity in sludge gas piping should not exceed 3.5 m/s. Pressure release valves are provided for controlling the gas in the digester by releasing gas pressure exceeding 200 to 300 mm of water and also preventing partial vacuum and possible cover collapse during rapid withdrawal of sludge or gas.
Performance of Digesters:
The following parameters of the digested sludge are indicative of good design of a digester:
(a) Approximate % of volatile solids reduced in digestion- 50%
(b) Gas production per kg of volatile matter destroyed- 0.9 m3
(c) pH of the digesting sludge- 7 to 8
(d) Methane content of gas produced- 60 to 70%
(e) Solid contents in the digested sludge for a feed sludge solids content of 4 to 6%
(i) Low Rate Digesters:
Primary- 10 to 15%
Mixed- 6 to 10%
(ii) High Rate Digesters:
Primary- 2.6 to 4%
Mixed- 2.0 to 3%
(f) Volatile acids concentration- 200 to 400 mg/l
(g) Grease- Practically absent
(h) Colour- Black
(i) Bicarbonate alkalinity- 2000 to 5000 mg/l
Note:
Gas production may also be reckoned in the following two ways:
(i) Gas production per kg of volatile matter added- 0.6 m3
(ii) Gas production per capita- 0.025 m3/day