Energy From Solid And Liquid Wastes - Viii

Lecture No: 18 Urban Waste to Energy from Landfill Biogas Projects and by Pyrolysis Plants 18.1. Introduction The urban waste is usually dumped in so called municipal landfills or municipal refuse dumps. These landfills are usually away from the city and occupy substantial land areas. Urban waste is transported by road trucks and is dumped into the landfills. Landfill waste gets fermented by natural bacterial decay (by anaerobic fermentation) and releases methane rich fuel gas. This gas is called landfill gas or refuse-tip-gas. Obtaining the methane rich fuel gas from landfills is the most economical and environmentally attractive method of obtaining energy from urban waste. Landfill Gas is being used as a renewable energy source in several countries in the world (Table 18.1). Table 18.1 Landfill Gas Project Sites (1998) Country

United states West Germany UK Sweden Italy Holland Denmark Canada France Norway Switzerland Australia Brazil India Chile

Outlet Electricity Purification Other Total Trials/ (pipeline) known schemes scheme generation/ * vehicle fuel application planned chp 22* 10 14 54 13

Boilers/ heating

Kilns furnaces

7

1

14+

5

17

---

7

43+

1

5

7

7**

---

2

19

6

2

---

4++

---

1

7

---

---

---

2

1

3

6

---

1

---

---

---

3

4

8

---

---

3

---

---

3

---

1

2

---

---

---

3

---

1

---

---

---

---

1

---

---

---

----

----

1

1

1

1

---

----

----

---

1

---

---

---

1

---

---

1

1

---

---

---

1

---

1

---

---

---

1

---

---

1

1

Total

---

---

---

1

---

1

1

33

15

57

13

31

146

32

* one scheme generates electricity and sells gas. + includes one research project. ** two schemes generates electricity and also gas for heating. ++ Four schemes are recorded as “ Boiler, CHP”. •

CHP = combined heat and power. Pyrolysis was tried for converting biomass from urban waste to energy. However,

the pyrolysis is used mainly for making wood-charcoal. 18.2. Applications of Landfill Gas Landfill gas contains predominantly methane (54% by volume). The landfill gas is used in following applications directly: --- As a fuel for burning in boilers (without purification) --- As a fuel for Kilns, Furnaces. The purified methane obtained from landfill gas is used in following applications ---- As a vehicle fuel. ---As a fuel for diesel engines. --- As a fuel for Diesel Engine, to produce electrical energy ---After upgrading, supplied as fuel gas to domestic consumers.

URBAN WASTE

RAW LANDFILL GAS

LANDFILL

FOR

FILTERS & PURIFIERS

GAS

FURNACES KILNS DOMESTI C FUEL

COLLECTIO FUEL FOR IC ENGINE ELECTRIC POWER

ENERGY FOR URBAN CONSUMERS

FROM

Fig 18.1 Application of Landfill gas (LFG). 12.3. Composition of Landfill Gas The land-fill gas is generated by the fermentation of organic matter dumped in the landfill. The process sis called anaerobic fermentation i.e. decomposition caused by (anaerobe, the microorganisms) without need of oxygen. This process is suitable for municipal manure. The process takes place at low temperatures up to 60°C and requires moisture. The gases produced vary in composition with time taken by the process (Fig.18.2). After a period of 2 months from starting, the landfill gas has mainly methane (52%) and carbon dioxide (46%). During initial periods other gases like oxygen, hydrogen, nitrogen etc. are released in different proportions.

Fig.18.2 Composition of the landfill gas changing with time During the decomposition, the temperature of the upper portion of the land-fill rises to about 60°C. Landfill gas is not a pure methane carbon dioxide mix. It has several other gases including some corrosive gases. For simple burning applications such as furnaces and kilns, the landfill gas is used without separation of methane and other constituents. For domestic cooking gas, the landfill gas is converted to compressed Natural GAS (CNG) or Liquid Natural Gas (LNG) by intermediate process. For use in vehicles as a fuel, the methane gas is separated form the total landfill gas and is purified to pure methane.

Lecture No: 19 Gas production- factors- Composition of landfill gas

19.1. Gas production If landfill gas is to be collected and managed either for energy production, flaring or for other reasons it is important to know the quantities and composition of the gas. In many cases predictions of future gas production and composition is desired. Of course the large variations in process conditions between landfills means that there is a great deal of uncertainty involved with estimating gas production, however, some general tendencies and tools will be presented in the following sections. 19.1.1 Gas quantities The total gas production from a typical enhanced bioreactor landfill will vary between 60-400 m3 per ton of waste with an average of 230 m 3 per ton (Gendebien, 1992). The annual gas production from a landfill is also very variable both as a function of time and between landfills. Gas production is in general controlled by:

•

Landfill temperature

•

Waste water content

•

Waste composition

•

Waste age

•

Landfill top cover ( entrance of atmospheric air )

Different materials have different biogas potentials and waste composition therefore has a very marked effect on gas production. It is possible to use Eq.5.2 to estimate the theoretical biogas potential, however, it is in general not a very good estimate of landfill gas production due to the mixed nature of the wastes and the variability in landfill process conditions. Gas potentials may be estimated if the waste composition is known using the data in Table 19.1. Gas potentials are typically lower in landfills than what would be expected if the materials were processed in a biogas reactor due to the intrusion of atmospheric oxygen into parts of the landfill.

Table 19.1. Gas production in Nm3 per ton dry matter for different biodegradable wastes. Data from Ehring (1984) Waste fraction Food waste Food waste with high bread content Grass clippings Leaves Food and yard waste with high wood content Newspapers Magazines Cardboard Mixed paper Sawdust MSW

Gas potential (55% methane) 191 – 344 135 176 60 45 – 60 120 100 – 225 317 65 – 242 30 160

Waste age has influence on gas production because it takes some time before newly deposited waste enters the methane phases. The gas production rate therefore will vary through time depending on the succession of the phases. If the gas production as a function of waste age is known for a particular landfill it is possible to estimate the landfill gas production from the landfill as a function of time. Here the data in Fig 19.1 will be used as an example, however it is noted that it is best to use data from landfills located in the same geographical area and are receiving wastes similar to the landfill in question. Also the fitted function in Fig. 19.1 may not fully represent gas production rate as a function waste age as data are only available for waste ages between 5 and 21 years. But for the sake of illustration the data is adequate. The total annual gas production rate at a given time T for a landfill receiving a waste quantity M each year can be estimated as: T

T

B(T) = ∫Mt . rT-t dt = ∑ Mi . r T-i 1

i=0

Where T is the landfill age (years) at the time of estimation, t is time (years), Mt is waste mass received at the landfill in year t and rT-t is the annual gas production rate per ton of waste with waste age T –t, i.e.., waste that was deposited in year t. considering a landfill that receives 50,000 tons of wastes each year and assuming that the annual gas production follows that in Fig. 7.8 ( with production rate equal to zero for waste ages

above 30 years) the total gas production will follow the curves shown in Fig 19.2 each curve represents different time spans for deposition of waste at the landfill. It is seen that the maximum gas production rate increases with the duration of the deposition period until about 30 years. If wastes are deposited at the landfill for more than 30 consecutive years the gas production will reach a maximum rate that will remain constant for some time depending on the length of the deposition period. The shapes of the curves depend upon the relation between the waste age and gas production rate (Fig.19.1). As discussed earlier the shape of the curves in Fig. 19.2 is a function of the shape of the gas production curve fitted in Fig. 19.1 and the results should therefore not be regarded as generally applicable to landfills.

Fig 19.1. Annual methane production in m3 per ton waste for 86 landfills. Bars indicate one standard deviation and the curve is fitted polynomial. In the cases of no standard deviation only one measurement was available

Fig 19.2. Calculated landfill gas production rate at a landfill receiving 50,000 tons of waste per year for different time periods

19.1.2. Gas composition Landfill gas is mainly composed of methane and carbon dioxide. The gas may also contain smaller amounts of other gases. When the gas is extracted from the landfill atmospheric air can be sucked into the waste and may be mixed with the gas. The gas can therefore contain smaller amounts of nitrogen and oxygen. Table 19.2 lists average values for typical landfill gas component concentrations. Landfill gas can also contain several other organic and inorganic trace components. Some of these compounds can be toxic or are carcinogens. Table 19.2. Typical landfill gas component concentrations Gas CH4

Range 30 – 65

Average 48

CO2

25 – 30

38

N2

5 – 30

12

H2

1–3

1

O2

0-5

1

The concentrations of these trace gases depends upon the composition of the waste deposited at the landfill. The most important trace gases are vinyl chloride, benzene, toluene, chloroform, and dichloromethane. These and other trace compounds

have been detected at European landfills. Typical concentration ranges of several trace gases found in landfill gases are shown in Table 19.3. Not all trace compounds found in landfill gas have been deposited at the landfill with the wastes. Several compounds are formed as a result of the microbial degradation of other compounds in the landfill. Table 19.3. Concentrations of common trace components found in landfill gas (Willumsen 1988). Compound

Concentration range (ppm)

Vinyl chloride (VC)

0.03 – 44

Benzene

0.6 - 32

Chloroform

0.2 – 2

Dichloromethane

0.9 – 490

Toluene

4 – 197

Xylene

2.3 – 139

Ethylbenzene

3.6 – 49

Chlorodifluoromethane

6 – 602

Dichlorodifluoromethane

10 – 486

Trichloroethylene (TCE)

1.2 – 116

Perchloroethylene (PCE)

0.3 – 110

Ethanol

16 – 1450

Propane

4.1 – 630

Butane

20. – 626

Carbondisulfide

0.5 – 22

Methanediol

0.1 – 430

Hydrogensulfide

2.8 – 27.5

Chlorine

1 – 10

Many chlorinated compounds such as solvents are degraded under anaerobic conditions by sequential dechlorination resulting in the formation of less chlorinated compounds. An example is perchloroethylene or tetrachloroethylene (PCE) that has been

used widely as a degreasing agent in the metal industry. This compound is transformed under anaerobic conditions to trichloroethylene (TCE) by microbial removal of one chlorine atom as illustrated in Fig. 19.3. TCE can under anaerobic conditions be transformed by further dechlorination into dichloroethylene (DCE),

which

can

be

degraded both anaerobically or aerobically. Under anaerobic conditions DCE is dechlorinated into vinyl chloride VC whereas it under aerobic conditions will be mineralized all the way to CO2, H2O and HCl. Vinyl chloride is not very degradable under anaerobic conditions and it will therefore disappear slower than parent compounds PCE, TCE and DCE, PCE and TCE have no proven toxic or carcinogenic effects in humans but VC has been shown to have both types of effects for humans and animals. Under anaerobic conditions VC can be transformed into ethane, however, it is degraded faster aerobically into CO2 and H2O. The products of anaerobic dehalogenation of PCE are all on gas form at normal temperature and pressure. They will therefore appear sequentially in both the landfill gas and the percolate if dehalogenation takes place in the landfill. Figure 19.4shows measurements of PCE and it dehalogenation products in landfill. It is evident in case (a) that no transformation takes place, as no intermediate products appear. This is likely because no PCE degrading microorganisms are present in the percolate. The slow decrease in PCE concentrations may be caused by evaporation of PCE from the percolate. In case (b) degradation starts some lag period whereas in cases (c) and (d) degradation proceeds rapidly. In case (b) Microorganisms capable of degrading PCE are likely present in the percolate, however, they have not been exposed to PCE prior to the experiment and therefore require some time to adapt to the new substrate. In cases (c) and (d) microorganisms seem to be well adapted to PCE and degradation of PCE and its intermediates is complete within 2 months. Most of the trace compounds listed in Table 19.3 will be present both in percolate, in the gas phase and adsorbed to the solids in the landfill. Some compounds adsorb very strongly and this will slow the degradation and transport of these compounds within the landfill. Strongly adsorbing compounds will therefore be more persistent in the landfill than non-sorbing compounds. Volatile compounds will be present in the landfill gas in high concentrations in the initial phases of the landfill life and concentrations will decrease as the compounds are removed with the gas or percolate.

Fig 19.3. Sequential dechlorination and transformation of PCE to ethane under anaerobic conditions

Fig 19.4. Sequential dehalogenation of PCE in four different landfill percolates

19.1.3. Gas utilization If the landfill gas is collected it can be used as a source of energy by conversion to electricity and heat. The most widespread method of energy production is to use the gas to drive a gas engine that is coupled to a generator that in turn produces electricity. Some plants also use part of the waste heat in the engine cooling water for heating nearby homes and business. This is mostly used in Europe where as it is not seen very much in USA. The waste heat normally amounts for at least 50% of the total energy contained in the input gas. Normal plants based on gas engines are from 350 to 1200 kW in size. At larger plants the energy conversion is based on gas or steam turbines. The gas is sometimes also used in boilers for heating water only. This can be used for heating. One reason that this approach is not widespread is that the price of electricity usually is higher than that of heat. Also electricity is normally easy to sell via the power grid. Landfill gas can also be used directly for instance in place of natural gas. The gas may be used in brick or cement production or in local water heaters. The gas can also be cleaned to match the quality of natural gas (mostly methane) by removal of primarily CO2. The gas can then be distributed via existing natural gas network. This means that new energy conversion plants are not needed. On the other-hand gas-cleaning plants are required. The technology is not widespread mostly due to economic reasons. At certain locations the gas is used in compactors, garbage trucks, buses or even cars. The economy in this type of gas utilization depends upon costs and taxes on other fuels in the region as well as the cost of constructing the gas collection and purification plant. The gas can also be used in fuel cells where it is converted to electricity. This technology is still in the development phase but it is expected that it will be used in the transport sector in the future. In 1997 there were 550 landfill gas collection plants with energy production worldwide (Willumsen, 1997). In Denmark there was 18 plants (1998) with a total production of 110.000 Nm3 landfill gas per day, equivalent to 5.1 MW energy (heat and power). This amounts to 0.1% of the total energy consumption in Denmark.

19.2. Percolate cleaning Percolate contains many different organic and inorganic compounds. The concentrations of these compounds often require treatment of the percolate before

discharge. Percolate from enhanced bioreactor landfills has typically much higher concentrations of organic materials and salts than regular wastewater. Concentrations may change rapidly following variations in precipitation and waste deposition patterns. The percolate flow will typically follow the precipitation pattern with a smoother variation. Old percolate is normally difficult to treat biologically as most of the degradable material has disappeared. Below is listed different methods for cleaning percolate. Cleaning in existing wastewater treatment plants can be done provided the percolate is diluted well with wastewater. There should not be more than 5% percolate in the mixture as there is a risk for overload with respect to organic matter, nitrogen or salts. Physical chemical cleaning can be used for instance for pretreatment prior to biological treatment or after biological treatment has been completed. Addition of precipitating agents can be used for removing inorganic compounds (salts, heavy metals, turbidity and color). pH adjustment and aeration can remove dissolved ammonia and aeration can also remove methane that can be an explosion hazard if the percolate is discharged into the sewer. Chemical oxidation with hydrogen peroxide has also been used for removal of iron and it has also been used to oxidize sulfide to remove odor and reduce corrosion problems in treatment facilities. Biological treatment of percolate is equivalent to wastewater treatment where organic matter, nitrogen and phosphorous can be removed. In old percolate a large fraction of the organic matter is difficult to degrade biologically and more advanced methods are necessary. Methods such as activated carbon filters, reverse osmosis and treatment with UV light can be used. Terrestrial methods such as irrigation,. Root-zone filters and infiltration plants have proven effective as final treatment methods (Christensen 1998) and are in some cases also useful as pretreatment methods. The advantage of terrestrial methods is their low installation and maintenance costs. There are however, problems with their efficiency in periods with low temperatures or high flow rates.

Lecture No: 20 Landfill Gas Collection System Landfill sit is usually a void, valley or a former quarry in which the urban waste is dumped. For example, a landfill site near Bedford, UK is a former brick quarry having original void volume of 10 million m3 and covering 100 km2 land area. The site receives about 1000 t of urban waste per day by rail, road from London. The gas collection system consist of Wells comprising vertical pipes of 80 to 120 mm diameter with holes in the cylindrical body. The wells are driven in the landfill. The well-pipes and collection pipes are of polythetene. Knockout drums are installed in the pipelines for removal of water. A typical landfill site has 20 to 40 wells and the collection pipe system. The wells are connected to manifolds and the gas is collected from the manifolds and piping system. 1. Gas Compression Equipment. The gas if filtered before and after compressor. The compressor increases the pressure required by the consumer device (e.g. diesel engine or a furnace). A single vane type gas compressor may be provided. A typical rating is 690m3/h of gas at discharge pressure of 1.3 bar. After compression the gas is passed through after cooler, baffle water separator, fine filter etc, before feeding to the gas consumer device. 2. Gas Purification. The landfill gas contains methane, carbon dioxide and many other impurities. Purification of this gas is complex and expensive. The cheaper and effective methods of purification have been developed recently (1990). These methods employ semipermeable membrane and molecular sieves. The purified methane can be used as a fuel for transport vehicles. 3. Energy Conversion Equipment. The landfill gas can be purified to pure methane and then used as a fuel for internal combustion engine. The Internal Combustion Engine can drive pumping sets. Alternatively, the Internal Combustion Engine can drive generator to produce electrical energy. For examples, a typical land-fill gas site may have one to four spark ignition type four stroke internal combustion engines. The engine drives a 350 kVA, 415 V generator.

The generator electric power is supplied to the distribution system at 6.6 kV via a step-up transformer in the substation. Technically, producing electricity from landfill gas project is more complex and demanding than use of gas directly as a fuel. Before admitting into the IC Engine, the gas should be cleaned, filtered and purified. The plant demands  More operational controls.  Greater security at outlet of Landfill Gas System.  Better operation and management of the total plant. For producing electricity sufficient gas should be available continuously to operate the plant throughout the year with a good plant load factor.

Lecture No: 21

Anaerobic Digestion The purpose of sludge digestion is to convert bulky, odorous sludges to a relatively inert material that can be rapidly dewatered without obnoxious odors. The bacterial process as summarized in Eq. 21.1,consist of two successive processes that occur simultaneously in digesting sludge. The first stage consists of breaking down large organic compounds and converting them to organic acids along with gaseous by-products of carbon dioxide, methane, and trace amounts of hydrogen sulfide. This step is performed by a variety of facultative bacteria operating in an environment devoid of oxygen, if the process were to stop there, the accumulated acids would lower the pH and would inhibit further decomposition by “pickling” the remaining raw sludge. For digestion to occur, second-stage gasification is needed to convert the organic acids to methane and carbon dioxide. Acid-splitting methane-forming bacteria are strict anaerobes and are very sensitive to environmental conditions of temperature, pH, and anaerobiosis. In addition, methane bacteria have a slower growth rate than the acid formers, and are very specific in food supply requirements. For example, each species is restricted to the metabolism of only a few compounds, mainly alcohols and organic acids, while carbohydrates, fats and proteins are not available as energy sources.

CO2, CH4 H2S Organic matter

CH4 Organic acids

Acid-forming bacteria

and Acid-splitting

----- (21.1)

CO2

methane-forming bacteria

Stability of the digestion process relies on proper balance of the two biological stages. Buildup of organic loading or a sharp rise in operating temperature. In either case,

the supply of organic acids exceeds the assimilative capacity of the methane-forming bacteria. This imbalance results in decreased gas production and eventual drop of pH, unless the organic loading is reduced to allow recovery of the second-stage reaction. Digesters may generate foam as a result of over-feeding. Accumulation of toxic substances from industrial wastes, such as heavy metals, may also inhibit the digestion problems is often difficult to determine. Monitoring volatile solids loading, total gas production, volatile acids concentration in the digesting sludge, and percentage of carbon dioxide in the head gases are the methods most frequently employed to give advance warning of pending failure. These measurements can also indicate the most probable cause of difficulties. Gas production should vary in proportion to organic loading. Volatile acids content is normally stable at a given loading rate and operating temperature. The percentage of carbon dioxide should also remain relatively constant. Monitoring digestion by pH, measurements is not recommended, since a drop in pH does not precede failure but announces that it has occurred. Table 21.1 lists the general operating and loading conditions for anaerobic digestion. Single-Stage Digestion A photo of a single-stage fixed-cover anaerobic digester is shown in Figure 21.1. the photo also shows ancillary equipment associated with digester heating: boiler, heat exchanger, and sludge recirculation piping. Raw sludge is pumped into the tank through feed pipes. Mixing pumps discharge at nozzles within the digester to keep the contents from stratifying. Without mixing, sludge separates, with a scum layer on top, a middle zone of supernatant water of separation underlain by actively digesting sludge, and a bottom layer of digested concentrate. A limited amount of mixing is also provided by withdrawing digesting sludge, passing it through a sludge heater, and returning it through the inlet piping. Supernatant is withdrawn from anyone of a series of pipes extended from the supernatant box. Digested sludge is taken from the tank bottom for dewatering. Highrate digesters are completely mixed the contents do not tend to separate or develop a clear supernatant, and the entire contents of the digester must be dewatered. For digesters designed with floating covers, the cover floats on the sludge surface, and liquid extending up the sides provides a seal between the tank wall and the side of the cover.

Table 21.1.General Operating and Loading Conditions for Anaerobic Sludge Digestion 98oF (36.7oC)

Temperature: Optimum PH:

General operating range Optimum

General limits Gas production

85o-99oF (29o – 37oC) 7.0 to 7.1 6.7 to 7.4

Per pound of volatile solids added

8-12 cu ft (230- 340 litres)

Per pound of volatile solids destroyed Gas composition: Methane

16-18 cu ft (450-510 litres) 65 to 69 percent

Carbon dioxide

31 to 35 percent

Hydrogen sulfide Volatile acids concentration

trace to 80 mg/l

General operating range Alkalinity concentration

200 to 800 mg/l

Normal operation Volatile solids loading

2000 to 3500 mg/l 0.02-0.05 lb VS/cu ft/daya

Conventional single stage First-stage high rate Volatile solids reduction

0.05-0.15 lb VS/cu ft/day

Conventional single stage

50 to 70 percent

First-stage high rate Solids retention time

50 percent

Conventional single stage First-stage high rate Digester capacity based on

30 to 90 days 15 to 20 days design

equivalent population Conventional single stage

4 to 6 cu ft/PEb

First-stage high rate

0.7 to 1.5 cu ft/PE

a

1.01b/cu ft/day = 16,000 g/m3 .d

b

1.0 cu ft = 0.0283m3

Fig 21.1. Photo of a single-stage fixed-cover anaerobic digester

Gas rising out of the digesting sludge is collected in the gas dome and is burned as a fuel in the sludge heater; often the excess is wasted to gas burner. The cover can rise vertically from the landing brackets to near the top of the tank wall guided by rollers around the circumference of keep it from binding. The volume between the landing brackets and the fully raised cover position is the amount of storage available for digested sludge; this is approximately one-third of the total volume. Digestion in a single-stage floating-cover tank performs the functions of volatile solids digestion, gravity thickening, and storage of digested sludge. When sludge is pumped into the digester from the primary settling tanks, the floating cover rises, making room for the sludge. Unmixed operation permits daily drainage of supernatant equal to approximately two-thirds of the raw sludge feed. Being high in both BOD and suspended solids, the withdrawn water is returned to the inlet of the treatment plant. Periodically, digested sludge is removed for dewatering and disposal. In large plants, digested sludge may be dewatered mechanically, however, in small installations it is frequently spread in liquid form on farmland or is dried on sand beds and hauled to land burial. Weather often dictates the schedule for land disposal, and, consequently, substantial digester storage volume is required in northern climates.

Typical operation lowers the cover to the landing brackets in the fall of the year to provide maximum storage volume for the winter. Fixed-cover digesters, where sludge is withdrawn as the digested sludge is displaced by the raw feed sludge, maintain a constant volume. Fixed-cover digesters require holding tanks, sludge lagoons, or other locations where displaced digested sludge can drain. Because the volume is constant and the cover is fixed, these digesters can be mixed by roof-mounted turbine mixers. The digester contents can be mixed using turbine mixers, externally mounted pumps, and gas mixing in draft tubes. Turbine, roof-mounted mixers are very efficient at mixing the entire tank contents. Rags can be removed by reversing the mixing direction. External mixing pumps can be mounted in draft tubes inside or outside of the digester, or in a pump piped to the digester. Figure 21.1 shows mixing pumps mounted outside of the digester tank. Pump mixing is also very effective, but may require multiple discharge points for large digesters. Gas mixing induces a flow within the draft tube to provide mixing. Mixing requirements may be expressed in terms to power input or turnover time. Typical values for power are 0.2 to 0.3-hp/1000 cu ft (0.005 to 0.008 kW/m 3). No allowance is made for the efficiency of converting power into mixing. Turnover time is calculated by taking the volume of the digester divided by the mixing flow rate. Typical designs are based on turnover rates of 30 to 60 min. Two-Stage Digestion In this process, two digesters in series separate the functions of biological stabilization from gravity thickening and storage shown in Figure 21.2. The first-stage high-rate unit is completely mixed and heated for optimum bacterial decomposition. These systems are available for installation in either fixed or floating-cover tanks. By using a floating cover digested sludge does not have to be displaced simultaneously with raw sludge feed as is required with a fixed-cover tank. In either case, however, the sludge cannot be thickened in a high-rate process because continuous mixing does not permit formation of supernatant. Actually, the discharged sludge has a lower solids concentration than the raw feed because of the conversion of volatile solids to gaseous end products. The second-stage digester must be provided with either a floating cover or gas dome and have provisions for withdrawing supernatant. The unit is often unheated, depending on the local climate and degree of stabilization accomplished in the first stage. By minimizing hydraulic

disturbances in the tank, the density of the digested sludge and clarity of the supernatant are both increased. Two-stage digestion may be advantageous in some plants, while conventional operation may be better in others. The determining factors include the size of the treatment plant, flexibility of sludge handling processes, method of ultimate solids disposal, storage capacity needed, and interrelated element of climatic conditions. For large plants with a number of digesters, series operation provides better utilization of digester capacity, but for small plants with limited supervision the conventional operation is frequently more feasible.

Fig 21.2. Two-stage anaerobic digestion is performed by two tanks in series. (a) The first stage tank on the left is completely mixed for optimum digestion. The second stage with a gas dome cover is for gravity thickening and storage of digested sludge (b) Photo of two-stage digesters at the Northeast wastewater treatment facility in Lincoln, Nebraska.

Sizing of Digesters Historically, conventional single-stage tanks have been sized on the basis of population equivalent load on the treatment plant. Heated digester capacity for a trickling filter plant processing domestic wastewater was established at 4 cu ft (0.11 m3) per capita of design load. For primary plus secondary activated sludge, the total tank volume requirement was increased to 6 cu ft (0.17 m3) per capita. These values are still used as guidelines of sizing conventional digesters for small treatment works. Total digestion capacity can be calculated for conventional single-stage operation using Eq. 11-42. Application of this formula requires knowing the characteristics of both the raw and digested sludges. V1 + V2 V=

T1 + V2 x T2

---------

(21.2)

2 where V = total digester capacity, gallons (cubic meters) V1 = volume of daily raw sludge applied, gallons per day (cubic meters per day) T1 = period required for digestion, days (approximately 30 days at a temperature of 85 to 90o F or 30 to 35oC) T2 = period of digested sludge storage, days The volume needed for the high-rate unit in a two-stage digestion system is based on a maximum volatile solids loading and minimum detention time. For new designs the generally adopted maximum allowable loading is 0.08 1b VS/cu ft/ day (1300 g/m3 .d) and the minimum liquid detention time is ten days. At these loadings and a temperature of 95oF, volatile solids reduction should be 50 percent or greater. No specific design criteria are established for second-stage tanks in high-rate systems, since thickening and digested sludge requirements depend on local sludge disposal procedures. Start-up of Digesters Anaerobic digestion is a difficult process to start because of the slow growth rate and sensitivity of acid-splitting methane-forming bacteria. Furthermore, the number of these microorganisms is very low in raw sludge compared with acid-forming bacteria. The normal procedure for start-up is to fill the tank with wastewater and to apply raw

sludge feed at about one-tenth of the design rate. If several thousand gallons of digesting sludge from an operating digester are used as seed, the new process cab be operational in a few weeks. However, if only raw sludge is available, developing the biological process may take months. Careful additions of lime added with raw sludge are helpful in maintaining the pH near 7.0, but erratic dosage can result in sharp pH changes detrimental to the bacteria. After gas production and volatile acids concentration have stabilized, the feed rate is gradually increased by small increments to full loading. Daily monitoring of this process involves plotting the daily gas production per unit of raw sludge fed, percentage of carbon dioxide in the head gases, and concentration of volatile acids in the digesting sludge.

Lecture No:22

Energy Recovery from sago effluent – design and application Physico-chemical characteristics of CSFE : The CSFE characteristics ere analysed throughout the period of study for different parameters and are shown in Table 22.1. Since the period of investigation extended from off season to peak season, a wide variation was observed in all the parameters. Table 22.1. Characteristics of CSFE SI.No. 1.

Parameters Total solids (TS),

Off-season 2100– 3620

Season 3900 – 4650

2.

mg/I Volatile solids (VS),

1700 – 2880

3320 – 3740

3.

mg/I Biochemical

556 – 2510

2650 – 4025

1110 – 4880

5515 – 7060

58 – 75

80 – 94

2550 – 2890

2900 – 3560

Oxygen Deman 4.

(BOD), mg/I Chemical Oxygen

5.

Deman (COD) mg/I Total Kjeldahi

6.

Nitrogen, mg/I Total organic

7.

carbon, mg/I Cyanide content,

0.3 – 0.55

0.65 – 0.9

8.

mg/I Threshold Odour

65 – 95

110 – 140

9. 10. 11.

Number (TON) PH BOD : COD ratio C : N ratio

4.5 – 4.9 0.5 – 0.51 44 – 38.5

4.9 – 5.3 0.48 – 0.57 36.3 – 37.9

Design and fabrication of anaerobic high rate reactors The methodology adopted for the design and fabrication of the anaerobic high rate reactors are outlined in the following section.

Selection of reactor configuration In consideration of the recommendations of Guiot and Vander Berg (1984) and Kennedy and Guiot (1986) that, a hybrid reactor can combine the advantages of upflow anaerobic filter and UASB, it was decided to design and fabricate an Upflow Anaerobic Hybrid reactor (UAHR). The expected advantages were an easy start-up and avoidance of possible complications with sludge granulation. Single phase operation was selected considering the recommendations of Lettinga and Hulshoff Pol (1980) that there is no reason to go for a two phase system in the case of soluble wastes.

Media placement and selection: It was decided to place the media on the upper 55 per cent of the reactor, height, leaving 10 cm at the top from the liquid surface (Young, 1991). Coconut shell was selected as the media to be compared with conventional PVC pall rings in the UAHR. The media size was selected considering the recommendations of Young (1991). The specific surface area around 100 m2 / m3 with a porosity over 85 per cent was the consideration for selection. The PVC pall rings selected had dimensions of 25 mm (diameter) x 22 mm (length). Coconut shells were broken into pieces such that it has an approximate specific surface area near to the recommendation. Coconut shells were available as half pieces and each half piece was broken into 2 to 3 pieces and screened to get a size 5 cm – 10 cm.

Estimation of media characteristics The procedures adopted for the estimation of specific surface area, porosity and bulk density for both media were as follows: The PVC pall rings were filled in a cylindrical vessel of 30 cm diameter and the bulk volume was measured for a known number of pall rings. The surface area of one pall ring was physically determined by linear measurements. Surface area of one pall ring(s), m2 x No. of pall rings (N) Specific surface area (Ss), = ---------------------------------------------------------------------m2 / m3

Bulk volume occupied by N number of pall rings, m3

To determine the specific surface area of coconut shells, 100 numbers of half shells were selected randomly and the surface area was linearly measured. The mean surface area of one half shell was then obtained. These shells were then broken into required size and were randomly filled in a cylindrical vessel of diameter 30 cm and the bulk volume occupied was found out.

Surface area of one pall ring(s), m2 x No. of pall rings (N) Specific surface area (Ss), = ---------------------------------------------------------------------m2 / m3

Bulk volume occupied by N number of pall rings, m3

To determine the porosity (P), the PVC pall rings as well as coconut shell media were filled in a cylindrical vessel with a predetermined volume of water. The media were filled in the vessel so that they are submerged and filled up to the water level. The new volume was noted down.

Initial volume of water Porosity of media (P) %

=

--------------------------------------- x 100 Volume after filling with media

The bulk density was estimated by finding the weight of a known volume for both types of media.

Dimensions of UAHRs The procedure adopted for arriving at the dimensions of the pilot scale UAHRs are given below. Design daily feed

=

50 l / day

Design HRT

=

4 day

Reactor liquid volume

=

50 x 4 = 200

The reactor height was selected considering the previous studies and ease for fabrication. Ozturk et al. (1993) used a 140 cm high UAHR with 60 per cent media. A height of 1.9 m was selected for the pilot scale UAHR. A cylindrical cross section was adopted since this is the most widely used one due to the enhanced uniformity in mixing and flow. Design media height, as percentage Of reactor height

=

55 per cent (Young, 1991)

=

10 cm

Clearance between top liquid Surface and media top level

1.9 x 55 Total height of media section

=

----------- = 1.045 m ~ 105 cm 100

The bottom 40 cm height of the reactor was designed in a conical shaped to enable mixing of feed and also easy sludge withdrawal (if necessary).

Total liquid volume, V = 0.2 m3 =

Vol. Of media filled cylindrical portion + Vol. of no-media cylindrical portion + Vol. of conical portion π

0.2

=

P

1

π

( --- D2x 1.05 x --- ) + (0.4 x --- x --- D2 ) 4

100

π + ( 0.45 x --- D2 ) 4

3

4

0.25465 D

=

√ ------------------------0.0105 P + 1.5833

Where. D

=

Diameter of the reactor, m

P

=

Porosity of media, per cent

But, a uniform diameter should be selected for both reactors, so as to get uniform hydraulic parameters. Hence average porosity (P) was taken for the design purpose.

Gas holder The Volume of gas holder was selected considering a biogas productivity of 3 lll feed. A gas volume measurement schedule of once daily was assumed at HRTs upto 8 day and as required thereafter. 200 Daily feed at 8 day HRT

=

-----

=

251

25 x 3 =

751

8

Gas production

=

Hence, a gas holder volume of 80 l was selected. The gas holder was to be designed in such a way that it provides a water seal like arrangement with provision for up and down movement. Hence, a clearance of 5 cm was provided in between the outer water jacket wall and the inner digester wall.

Diameter of gas holder

=

D + 0.05 m

Diameter of water jacket

=

Volume of gas hbolder, V (g) =

D + 0.05 + 0.05 = D + 0.1 m 0.075

= (π (D+0.05)2/4) x H(g)

Where, H (g) is the effective gas holder

Gas production Gas production rates are the most important indicators of reactor performance for anaerobic reactors. Table 4.25 shows the gas production data of the UAHRs at PSS periods of various HRTs. The mean daily gas production of reactor 1 increased from 58.6 l (15 day HRT) to 458.5 l (1 day HRT) showing 7.8 times increase, while reactor 2 had an increase from 58.1 l to 474 l (8.2 times). At the same time specific gas production (l/kg TS) decreased from 908.5 l (at 15 day HRT) to 574 l (at 1 day HRT) for reactor 1. For reactor 2, the corresponding figures were 844.5 and 556 l/kg TS. The per cent decrease over initial values were 6.8 and 34.1, respectively for reactors 1 and 2. The trend of variation over different HRTs are illustrated in Fig.4.27. The maximum specific gas productions obtained in this study were 3.5 and 3.3 fold higher than the highest value of 434.8 l / kg TS

reduced

obtained in batch digestion experiment for reactors 1 and 2,

respectively. Specific gas production A maximum specific gas production of 1108 l/kg VS and 1030 l/kg VS were obtained for reactors 1 and 2, respectively at the longest HRT of 15 day. The corresponding minimum values were 725 l/kg VS and 703 l/kg VS at the shortest HRT of 1 day. Chawla (1986) reported that a maximum gas production of 1000 l/kg VS is achievable, assuming a VS reduction of 50 per cent. In the present study, the VS reduction corresponding to the maximum specific gas production (reactor 1 at 15 day HRT) was 76.2 per cent. Hence the maximum value of specific gas production was much higher than the aove reported value. However, the maximum specific gas production expressed as l/kg VS

destroyed

(1454 l) was lower than the maximum value of 2000 l/kg

VSdestroyed reported byChawla (1986). Lo and Liao (1986) also could get a biogas yield of 1048 l/kg VS for a mixture of screened dairy manure and winery waste which is similar to the results of the present study.

Specific gas productions in terms of BOD and COD also exhibited similar pattern of steady decrease wich is depicted in Fig.4.27. The maximum values (at 15 day HRT) were 1121.4 l/kg BOD and 604 l/kg COD for reactor 1. The corresponding values were, 1125 l/kg BOD and 561.7 l/kg COD for reactor 2. The minimum values obtained at

Table 22.2. Gas production at different HRTs

Parameters Mean daily gas production, 1 Specific gas production, 1/kg TS Specific gas production, 1/kg VS Specific gas production, 1/kg BOD Specific gas production, 1/kg COD Specific gas production, 1/1 feed Specific gas production, 1/m3 reactor

Reactor 1 Reactor 2 Reactor 1 Reactor 2 Reactor 1 Reactor 2 Reactor 1 Reactor 2 Reactor 1 Reactor 2 Reactor 1 Reactor 2 Reactor 1 Reactor 2

15 58.6 58.1 908.5 844.5 1108 1030 1121.4 1125.0 604.0 561.7 3.9 3.63 260.4 242.1

11 80 78 903.4 826.3 1123.6 1028.2 1197.2 1094.3 599.6 548.1 3.9 3.6 356 325

8 118.6 125 919.4 901.9 1140.4 1113.1 1064.9 1052.0 600.4 593.2 4.24 4.16 527 520.8

HRT 6 153.7 162 881.3 871 1101.8 1088.7 1019.6 1007.5 583.9 576.9 4.1 4.05 683 675

4 194.3 198.3 822 787 1028 981 1053.1 1007.6 583.5 558.3 3.45 3.3 863 826

2.5 272.2 271.7 775 726 993 930 1050.2 982.7 548.4 513.2 3.02 2.83 1210 1132

1.67 354.7 356.5 723 680 909 857 1042.6 982.4 536.2 505.2 2.62 2.47 1576 1485

1 458.5 475 574 556 725 703 825.0 799.6 449.8 436.0 2.04 1.98 2038 1975

Volumetric biogas production The maximum biogas productivity obtained per litre of CSFE was 4.24 l/l and 4.16 l/l (at 8 day HRT) for reactors 1 and 2, respectively (Table 22.2.). The corresponding minimum values were 2.04 l/l and 1.98 l/l at 1 day HRT. From 15 to 8 day HRT period, the biogas productivity increased due to the increased TS content of 4620 mg/l as against 4300 mg/l at 15 day HRT. There after the values showed a decreasing trend as depicted in Fig.4.28. This could be attributed to the decrease in reactor performance at increased loading rates as well as the lowering in strength of the feed. Fernandez (1999) could get 6 m 3 CH4/m3 of wastewater wile treating citric acid factory effluent and this high volumetric productivity compared to the present study might be due to the high strength of the wastewater. The volumetric gas production (l / m3 of reactor volume) steadily increased from 260.4 at 15 day HRT to the maximum values of 2038 and 1975 l/m3 (1 day HRT) for reactors 1 and 2, respectively. The increase of volumetric biogas production was gradual upto 4 day HRT and drastic from 4 day to 1 day HRT due to the sudden increase of HLR and OLR.

TS and VS reductions The TS and VS reduction as per cent of influent concentration is shown in Table 4.26. The maximum TS reduction of 60 per cent and 59.3 per cent occurred at 15 day HRT, for reactors 1 and 2, respectively. The minimum TS reductions were 33.8 per cent and 32.4. The TS reduction was almost steady upto 8 day HRT and there after it showed a steady decreasing trend. The VS reduction also showed a similar trend. The maximum VS reductions were 76.2 and 75.9 per cents at 15 day HRT for reactors 1 and 2. The minimum values were 49.5 and 48 per cent at 1 day HRT. 1 day HRT were 825 l/kg BOD and 449.8 l/kg COD for reactor 1 and 799.6 l/kg BOD and 436 l/kg COD for reactor 2. It became evident from these parameters that reactor 1 was superior to reactor 2 at PSS of all HRTs with respect to specific gas productions expressed in terms of TS, VS, BOD and COD. Dararatana (1991) got a very high specific biogas production of 0.98 m3/kg CODremoved from cassava alcohol slop. The maximum specific biogas production of 0.604 m3 / kg COD obtained in this study (reactor 1 at 15 day HRT) is equivalent to 0.628 m 3/kg COD

removed

which is much lower than the above reported value. According to the reports of FAO (1983), beef cattle wastes can produce 0.56-0.71m3 CH4/kg VS

added.

Hashimoto (1983) reported a value of 0.49 m3 CH4/kg VS

added

for swine

manure. Sarada and Nand (1989) could get 0.60 m 3 CH4/kg VS from tomato processing waste while Viswanath and Nand (1994) got 0.53 m3 CH4/kg VS from silk industry wastes. In this study, a methane productivity of 0.78 and 0.74 m3 CH4/kg VS were obtained for reactor 1 and

2, respectively at 15 day HRT. These values decreased to 0.47 and 0.45 m 3 CH4/kg VS for reactors 1 and 2, respectively at 1 day HRT. The values of specific gas production obtained at the longest HRT of 15 day were higher than the values reported by these workers and the values obtained at the shortest HRT of 1 day were lower or nearer to the ranges reported by other workers. This is due to the difference in substrate and operating conditions. CSFE was a highly biodegradable material with readily hydrolysable constituents like starch. The very dilute nature (maximum TS, 4650 mg/l) of the feed stock also helped in the well mixing of substrate solids which favoured easy digestion. 15 day was a rather long HRT at which very high biodegradation efficiency could be achieved. BOD and COD reduction

The BOD and COD reduction of the UAHRs at PSS periods various HRTs are shown in Table 22.3. A very high BOD reduction of 99 per cent and 98.9 per cent for reactors 1 and 2 were obtained at the longest HRT of 15 days. The maximum COD reduction were 96.2 and 96 per cents for reactor 1 and 2, respectively. Upto 6 day HRT, both the reactors exhibited steady performance irrespective of the influent concentrations. There after a decreasing trend was observed due to the increased loading rated. Ths change was sharp between 4 day HRT and 2.5 day HRT because of the drastic change in HLR (60 per cent increase). The lowest reduction of 78.9 and 77.4 per cent BOD and 77.4 and 76 per cent COD occurred at 1 day HRT. Reactor 1 was found superior to reactor 2 in BOD and COD reduction at all HRTs.

Reactor performance at PSS of different HRTs Parameters TS

Reactor

Reduction, 1 Reactor % 2 VS Reactor Reduction, 1 Reactor % 2 BOD Reactor Reduction, 1 Reactor % 2 COD Reactor Reduction, 1 Reactor % 2 TVA Reactor Reduction, 1 Reactor % 2 TON Reactor Reduction, 1 Reactor % 2 CH4 Reactor content of 1 Reactor biogas, % 2

HRT 15 60

11 57.2

8 56.7

6 55.5

4 50.5

2.5 46.1

1.67 40.3

1 33.8

59.3

56.8

55.6

55.1

49.5

44.9

38.1

32.4

76.2

75.3

75.9

70.6

65.7

59.9

53.8

49.5

75.9

74.7

75.4

69.9

64.5

58.6

52.4

48

99

98.6

98.8

98.6

95.6

87.5

83

78.9

98.9

98.3

98.3

98.2

94.4

85.8

82.4

77.4

96.2

96.4

96.2

96.1

93.2

86.3

81.8

77.4

96

96.3

95.8

95.2

92

85

80.5

76

97.5

96.7

96

95.5

94.5

92.5

91.8

90.3

96.9

96.1

95.6

95.5

93.9

91.9

91.1

89

97.1

96.1

95.2

91.7

89.2

80

66.3

50

96.4

94.8

94.5

93.1

88.5

79.1

63.2

37.8

70

71

72

72

70.5

68

66

65

72

73

74

73

71

68.5

65

64