ME597 Research Paper Preliminary work for development of an efficient anaerobic membrane bio-reactor Joel Torres-Crespo, PE November 30, 2009

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Contents I. Introduction............................................................................................................................. 3 II. Define and understand the anaerobic process......................................................................... 4 III. Puerto Rico Waste water discharges................................................................................... 5

A. Rum distilleries and brewery waste .................................................................................... 5 1. Composition of waste ..................................................................................................... 5 2. Waste strength................................................................................................................. 5

B. Pharmaceutical plants waste ............................................................................................... 6 1. Composition of waste ..................................................................................................... 6 2. Waste Strength ................................................................................................................ 6

C. Municipal waste water ........................................................................................................ 7 1. Composition of waste ..................................................................................................... 8 2. Waste Strength ................................................................................................................ 8

D. Food processing waste ........................................................................................................ 9 1. Composition of waste ................................................................................................... 10 2. Waste Strength .............................................................................................................. 10

E. Water Hyacinth (Eichhornia Crassipes)............................................................................ 10 V. Evaluate the energy content of the waste discharges defined............................................... 11 VI. Identify means or processes to increase the efficiency of anaerobic processes................ 12

Mechanical Pretreatment ...................................................................................................... 12 Thermal Pretreatment............................................................................................................ 13 Chemical Pretreatment.......................................................................................................... 14

VII. Evaluate which pretreatment methods are more efficient depending on the waste. ......... 14 VIII. Anaerobic digester plant energy from combustion........................................................... 19 IX. Conclusions....................................................................................................................... 20 X. References............................................................................................................................. 20

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I. Introduction

Anaerobic digestion has been used primary on the waste sludge treatment on wastewater

treatment plants but now, several uses on agricultural and industrial has been taking much

attention as you will further see.

Nowadays several environmental and energy issues are being address regarding water

pollution and renewable energy sources. Fossil fuels are depleting and we need to come up with

alternatives in renewable energy as solar, wind or biomass energy. That is why I want to give

this preliminary work needed for developing an efficient anaerobic membrane bio-reactor more

emphasis on the biogas production. The reason I will follow this approach is because I consider

biomass and biogas production as an alternative energy source for the future.

Before beginning to start gathering information about the kinds of waste feeds available for

the anaerobic process I needed to understand more deeply the complete anaerobic process and

how each step contributed to the final products. This knowledge of the process will help me to

identify what needs to be improved to enhance the complete process and to be more efficient.

The complete anaerobic process will be explained on section II.

After getting to know the anaerobic process, I then will proceed to identify possible waste

material candidates on the island of Puerto Rico that can be processed in an anaerobic digester.

The identification of each possible candidate will include a brief description of the

material, its composition and its waste strength and characteristics. The identification of the

possible candidates will also help identify which pretreatment methods can be applied on the

hydrolysis step to make the anaerobic process more efficient.

The preliminary work will conclude with an overview of the approximate amount of

energy that can be obtained from the biogas combustion.

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II. Define and understand the anaerobic process

The anaerobic digestion can be defined as the fermentation process in which organic

material is degraded and biogas (composed of mainly methane and carbon dioxide) is produced4.

In Germany changing legislative restrictions and acts on granting priority to renewable energy

sources offer incentives that will greatly impulse this technology . In 2005, approximately 2,400

biogas plants in Germany with a total capacity of 450MW demonstrate the technical and

economical feasibility of Anaerobic Digestion of organic wastes and waste water5.

Anaerobic digestion process

The first step in the Anaerobic digestion of organic matter is hydrolysis. The hydrolysis

process begins when hydrolytic bacteria break down the complex structures found on the waste

stream, for example proteins or lipids, into more simple water soluble compounds like amino

acids, sugars or fatty acids. This step is the one of the most time limiting process in anaerobic

digestion only next to methanogenesis. We will further discuss additional techniques that can

reduce the amount of time for this step.

After the hydrolytic bacteria break down the complex organic molecules, now the

resulting compounds are ready to be treated by other types of bacteria.On the Acidification

process, fermentation acidogenic bacteria further broke down into water soluble organic

compounds like short chained fatty acids like butyrate, alcohols, H2 and CO2.

In acetogenesis, just the step before the convertion to methane gas, the end products of

the microbial metabolisms are converted into short chained volatile fatty acids, specially acetate

and CO2. These acetogenesis bacteria need to have a low H2 concentration atmosphere in order to

convert the end products produced on the hydrolisis and acidification steps into acetate and CO2.

The last step of the anaerobic process is the more characteristic one. Is the process of were

the methanogenic bacteria convertings the acetates, H2 and CO2 into biogas. Biogas is a mixture

of methane (CH4, 50-85% by volume), CO2 (15-50% by volume) and some trace gases like water

vapor, H2S or H2.

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III. Puerto Rico Waste water discharges

A. Rum distilleries

In Puerto Rico, one of the biggest industries is rum distilleries. On the island there are two

big distilleries, Bacardi Corporation on the north part of the island and Destileria Serralles on the

south. Bacardi corporation is one of the biggest distilleries in the world. They produce an

average of 200 millions cases of rum per year. This kind of manufacturing activity creates lots of

waste stream with high organic loadings that need to be treated. The effluents of these

manufacturing activities mainly consist of cooling water from stills, fermentation vats, hot spent

wash, and water used for cleaning the fermentation vats and distillery1. These effluents usually

have a Biological Oxygen Demand (BOD) close to 32,000 mg/l, making it a very high organic

effluent.2

1. Composition of waste

The main component in Rum distilleries that contribute to the high organic load of waste

water are rum slops. Rum slops are made up of sugar, organic acids, amino acids, proteins,

polysacharrides, and inorganic salt complexes. 6

2. Waste strength

Bacardi Corporation performed a case study of a energy production project using anaerobic

processes to create biogas so it can be use as energy input to the plant.6 On this case study they

presented the actual scenario of the composition of the “fuel” that was going to be supplied to the

anaerobic digester. On this case the fuel input mainly is composed of rum slops. The strength of

the rum slops is described on Table 1.

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Parameter Unit Range

BOD5 mg/l 36,000-42,000

COD mg/l 80,000-105,000

TSS mg/l 3,000-8,000

TS % 7.5-11.0

pH SU 4.2-5.0

Alkalinity(CaCO3) mg/l 600-1,700

Volume of Acids(CH3CO2H) mg/l 4,000-7,000

Specific Gravity -- 1.02-1.05

Table 1: Bacardi case study - Waste strength

B. Pharmaceutical plants waste

One of the wastes I want to analyze is the wastewater generated by pharmaceutical plants in

Puerto Rico. I tried to gain data from several plants on the island but they were not quite

accessible due to intellectual property issues regarding their manufacturing. I then did a search of

data obtained from other researchers on plants similar to the ones in Puerto Rico.

1. Composition of waste

The composition of the waste stream supplied by pharmaceutical companies depends on

the product manufactured on the site. For example, a pharmaceutical plant that manufactures the

antibiotic erythromycin creates one of the highest organic load wastes. These waste streams

mainly consist of residual amounts of antibiotics, extraction solvents, grain flours, sugars, protein

and nutrients3. This kind of waste stream usually has very consistent waste characteristics, very

important for anaerobic digestion.

2. Waste Strength

For example, an Abbott fermentation plant in Chicago, Illinois identical to the one in

Puerto Rico had the following characteristics; See table 2.

Another pharma plant is Puerto Rico, Smithkline, has the following characteristics on their

waste water; see table 3. It can be seen from these tables that the composition of the wastewater

directly depends on the products manufactured.

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Parameter Unit Average Value

Flow Mgd .457

COD mg/l 8683

BOD mg/l 4131

TSS mg/l 2268

Table 2 : Pharma Fermentation plant data

Parameter Unit Average Value

Flow Mgd .59

COD mg/l 381

BOD mg/l 166

TSS mg/l 555

Table 3:Pharma finishing plant data

C. Municipal waste water

The Puerto Rico Aqueduct and Sewers Authority (PRASA) is the public corporation in

charge of the supply of fresh water and treatment of used water on the island of Puerto Rico.

They have 60 waste water treatment plants all around the island. They include a combination of

Primary, Secondary and terciary treatments. Figure 1 depicts the location and kind of waste

water treatment plants are around the island.

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Figure 1:Wastewater Treatment and Filtration Plants in Puerto Rico

1. Composition of waste

The composition of these waste streams mainly consist of rainwater, feces, urine, paper,

trees and leaf clippings, residential food residues, etc.

2. Waste Strength

Eng. Juan Perez, auxiliary director of compliance and quality control from the PR

Aqueduct and Sewers Authority, provided operational data they submit to EPA showing several

operating parameters from the WWTP in Puerto Rico. The excel report contains important

parameters, for example, BOD, TSS, flow, percent removals for each WWTP in Puerto Rico for

that last year.

From it I obtained the information I considered the most important and tried to gain a

sample from each part of the island.

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North South West East Center

Town BOD5 mg/l Town BOD5

mg/l Town BOD5 mg/l Town BOD5

mg/l Town BOD5 mg/l

Arecibo 148 Santa Isabel 240 Cabo Rojo 282 Fajardo 125 Jayuya 227

Barceloneta 126 Ponce 181 Mayaguez 241 Humacao 133 Corozal 347 Puerto Nuevo 161 Yauco 257 Aguadilla 212 Yabucoa 191 Barranquitas 193

Table 4 : Influent BOD5 for several municipalities in Puerto Rico

North South West East Center

Town Flow MGD Town Flow

MGD Town Flow MGD Town Flow

MGD Town Flow MGD

Arecibo 5.35 Santa Isabel 1.89 Cabo Rojo .281 Fajardo 3.03 Jayuya .446

Barceloneta 5.33 Ponce .379 Mayaguez 9.81 Humacao 5.26 Corozal .610 Puerto Nuevo 58.49 Yauco 1.528 Aguadilla 4.92 Yabucoa 0.984 Barranquitas 0.340

Table 5 : Flow MGD for several municipalities in Puerto Rico

North South West East Center

Town TSS mg/l Town TSS

mg/l Town TSS mg/l Town TSS

mg/l Town TSS mg/l

Arecibo 205 Santa Isabel 246 Cabo Rojo 1819 Fajardo 128 Jayuya 207

Barceloneta 209 Ponce 193 Mayaguez 400 Humacao 146 Corozal 1759 Puerto Nuevo 295 Yauco 226 Aguadilla 500 Yabucoa 265 Barranquitas 473

Table 6: Influent TSS for several municipalities in Puerto Rico

D. Food processing waste

The are several food processing industries in Puerto Rico that highly contributes to the

organic loads found on the wastewater treatment plants. The main industries involved on are the

vegetative, poultry, pig and dairy.

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1. Composition of waste

The waste found on several food processing farms depends on the activities on the facilities.

For example, poultry slaughter streams usually contains organic wastes from broilers. These

wastes are defined as organic biodegradable waste with moisture content below 85–90 % 9 with

high amounts of blood on their streams. Poultry blood accounts for approximately 2% of the

bird’s weight and blood 95% content are proteins. 8

On the other hand, poultry and livestock manures consists of feces, urine, hair or feathers,

spilled water and feed, water used for flushing gutters and cleanings process equipments, etc.7

Piggery waste is characterized by a high content of organic matter consisting of a mixture of

manure (feces and urine) and food wastage such as swill and sugar cane molasses10.

2. Waste Strength

Poultry waste has the following waste strength characteristics: It has a water content of

approximately 77.5% with a density of approximately 1,102 kg/m3. These wastes have an

average COD and BOD5 of, 12,100 mg/l and 5,900mg/l respectively. Also it has total solids of

8,280 mg/l and volatile solids in the amount of 5,370 mg/l. 11

Piggery waste has very high organics contents with a total chemical oxygen demand of

approximately 10,189 mg/l. The total organic carbon is 4000 mg/L in average. The mean values

of: total solids is 7,210 mg/l, total volatile solid is 5122 mg/l, and total suspended solid is 1637

mg/l, respectively. 10

E. Water Hyacinth (Eichhornia Crassipes)

During the research of different waste types in Puerto Rico that were ideal candidates for

anaerobic digestion I found out about Water hyacinth and its characteristics. Water hyacinth is a

free-floating perennial plant that can grow to a height of 3 feet. The dark green leave blades are

circular to elliptical in shape attached to a spongy, inflated petiole. Underneath the water is a

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thick, heavily branched, dark fibrous root system. The water hyacinth has striking light blue to

violet flowers located on a terminal spike.23

This kind of aquatic plant has become a menace to navigation not only in Puerto Rico but in

other parts of the United States and South America, because of its ability of obstructing channels

with high amount plants that prohibits the free flowing of boats. The good news about the water

Hyacinth is that it has been identified as a good candidate for anaerobic digestion because if it’s

good methane production. Therefore if the amount of water hyacinth continues increasing and

clogging navigation passageways then it can be used in a more efficient way instead of just

getting rid of it.

V. Evaluate the energy content of the waste discharges defined.

The energy content of the waste analyzed on section IV highly depends on the state that the

waste is found. For example, highly diluted waste obtained from manure offers lower energy

density (energy per unit of mass) than waste found in a dried manner. Here are some examples of

energy contents found in literature that confirm this statement:

-Based on the data obtained from existing poultry manure anaerobic digester, it can be calculated

that the amount of energy in poultry manure is approximately 5,200 kJ/kg of poultry manure for

the production of biogas. 13

-When industries prefer co-firing of the dried manure the energy contained on one pound of

dried cattle manure is approximately 13,400 ± 510 kJ/kg.25

The University of Vienna offers an online database26 were they gathered information

regarding biomass. They offer energy information of different kinds of wastes. Here are

examples of heating values of different waste types described above of section III:

-Animal waste higher heating value 17,173 kJ/kg

-Municipal domestic and industrial waste higher heating value 17,300 kJ/kg

-Waste from brewery industries higher heating value 22,200 kJ/ kg

This energy information describes the amount of energy that the waste has, and that will be

released based on direct incineration of the materials. Sometimes this is done but is not the

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efficient way to do it because this incineration has lots of residues like ash that then becomes an

issue its disposal.

On section VII an energy balance will be performed on the combustion performed on a

typical biogas incineration.

VI. Identify means or processes to increase the efficiency of anaerobic processes

On the anaerobic process, it has been proven that the methanogenic reaction is the most rate-

limiting step but when considering particulate substrate like solid waste the step of hydrolysis on

anaerobic digestion is identified as one of the rate-limiting steps on the whole process. That is

the reason why several pretreatment techniques have been developed to further reduce the

amount of time needed for this step. For example, several treatments like thermal, alkaline,

ultrasonic or mechanical disintegration have been studied and they can accelerate the hydrolysis

process and reduce the particle size. These improvements will eventually increase the efficiency

of the anaerobic process, not only on the organic waste reduction but also on the biogas

production.

Each pretreatment process does not act equally on all organic materials being processed via

anaerobic digestion. That is the reason why these is a need to analyze how each pretreatment

method can increase the yield of biogas production depending on the waste type and also on the

additional components found on the wastewater.

Mechanical Pretreatment

The mechanical pretreatment process consists of reducing the particle size of the solid

portions of the waste stream to be treated. This reduction is particle size is performing by shear

forces exerted on the solid materials by some kind of knife like equipment. Several research have

demonstrated that the production of biogas on anaerobic digester in inversely proportional to the

substrate particle average diameter.

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The reduction of particle size on the solid substrates of the waste increases the surface

area of the materials, thus increasing the degradation of the material and increasing the digestion

process.

One method used on the existing anaerobic digesters is the use of macerators pump.

Macerator pumps if a device designed to reduce the particle size of materials flowing through it.

There are several models but one typical arrangement I like a centrifugal pump with the impeller

specially designed to reduce the solid influents into smaller particle size. This reduction in

particle size has been shown that the biogas yield can be increased form 5-25%14. Another factor

very attractive associated with this kind of pretreatment is the low cost of implementation and

operation.

There are several other pretreatment methods that can be categorized as mechanical

pretreatments, for example ultrasonic pretreatment:

Ultrasonic pretreatment of sludge can be categorized very similar as the mechanical

pretreatment explained before. The principle of operation of this pretreatment method is that the

sludge is exposed to an ultrasonic source of energy and this exposure will rupture the microbial

cell walls and membranes and will release the intracellular organics in the bulk solution17. This

phenomenon has been demonstrated that enhances the overall digestibility of the sludge. This

sonification energy generates cavitations (implosion) processes in liquids giving rise to local

high temperature hot spots of over 1000C and pressure increases of 500 bar.18

Thermal Pretreatment

The thermal pretreatment of the sludge before hydrolysis consists of elevating the

temperature of the mixture to a predetermine degree in order in increase the solubility of the

sludge. This pretreatment is usually used as a conditioning process for raw or digested sludge

and improves the dewaterability of the processed waste.14

A study performed by Dote et al (1993) explained that the effect that the thermal energy

and temperature increase on the sludge is an increase on the liquidization of the solid materials.

This liquidization is caused by the rupture of the cells of the microorganism that make up the

major part of the sewage and the water phase is then available to suspend the cellular debris thus

reducing the viscosity. 15

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There are several disadvantages of this pretreatment method documented on the

literature. For example there is a problem with the odors produced. Also there is also presence of

corrosion and organic fouling of heat exchanger tubes. This pretreatment method is also high

energy consumption.

The advantages of this methods include improve dewaterability, increase biodegradability

and a reduction of strength in the liquid streams and substrate sterilization 14.

Chemical Pretreatment

Another methods studied in order to increase the efficiency of the anaerobic process is

the used of chemical agents. Several studies have proposed the use of acids or alkali for the

chemical pretreatment of sludge before the anaerobic process. These studies showed that usually

alkali agents are more compatible with the anaerobic digestion process because of the inherent

properties of the anaerobic conversion; the pH needs to be adjusted by increasing alkalinity.

The effect of alkali pretreatment of high cellulose materials yields a chemical reaction that

consists of a saponification of esters of uronic acid associated with xylan chains.14

VII. Evaluate which pretreatment methods are more efficient depending on the waste.

In this section, most of the Puerto Rico waste types mention on section III, will be evaluated

and decide, based on research, which pretreatment method make a more efficient yield of biogas.

Also optimal pretreatment characteristics will be identified in order to make the system more

efficient. There were several waste types in which overall research has not or has little research

done. Therefore, the most available ones will be analyzed based on the information found from

different sources.

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Cow waste

On all the research articles I found that investigated pretreatment methods for Cow waste

anaerobic digestibility, Thermo chemical pretreatment has been shown to be the most effective

one.

On the article from Jin et al 2009, they performed an experiment using dairy manure and

used microwave based thermo chemical pretreatment on the sludge. They also compare

conventional autoclave heating of the raw material. The raw manure was diluted with distilled

water to a solid content of 6.67%(w/w) and mixed with different chemicals like NaOH, CaO,

H2SO4 , H202 and HCl.

The results from the study show that the alkali chemicals (NaOH and CaO) improved the

methane production compared to acid chemicals. Also they showed that between the two heating

methods used, conventional autoclave heating and microwave based heating, the last one had

better results on methane production. They suggest that the microwave based heating develops a

better anaerobic digestibility.

They also optimized two microwave heating parameters, heating temperature and heating

time based on methane production. Their experiment optimal methane cumulative production

(443 ml per g of dry matter) was obtained at a heating temperature of 147C and a heating time of

25.3 min.

They also found that adding 0.07g per g of dry matter of NaOH was the optimal alkali

chemical addition for methane production.

Swine waste

Swine waste has been one of the major environmental concerns when dealing with this

kind of industry in Puerto Rico. This waste usually is used as fertilizer but has been a recent

issue due to contamination related to these kinds of waste. That is the reason why the

investigation of treatment of this waste type needs to be increase.

Based on research, this kind of waste type untreated is not an ideal anaerobic process

candidate because of it low biogas production characteristics21. That is the reason why

improvements and pretreatments of this waste need to be done in order to increase the biogas

production and making it more attractive for anaerobic digestion.

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Several investigations have been conducted to improve the effectiveness of the anaerobic

digestion process using swine manure as feed. One of the most promising results were obtained

from an investigation done by González-Fernández et al 2008, where they experimented three

pretreatment methods as explained on section V. They investigated the results of biogas

production based on three pretreatment methods: mechanical, chemical and thermal. The results

yielded that the thermal pretreatment was the most efficient one regarding the biogas production.

The results were an increase on 35% on the amount of biogas obtained compared to the untreated

feed. The untreated feed had a biogas production of 0.195 L CH4 per g of CODadded. The

thermally treated feed at 100C for 20 min had a methane production of 0.263 L CH4 per g of

CODadded.

Poultry waste

As with the other food processing waste, poultry manure has been identified as another

important candidate for anaerobic digestion for treating wastewater and also as a renewable

energy source due to the generation of biogas. Poultry manure waste has not been properly

studied on the effects of all kinds of pretreatments to efficiently identify which method is the

most effective one. I have found that the researchers usually concentrate on the slaughter

residues of poultry, because these materials have more resources to obtain high methane

production yields. I gathered information from an investigation performed by Salminen et al

200322, where they documented the effects of pretreatments on the slaughter residues of poultry.

They described that the wastes they treated were mainly poultry offal, blood and bone meal on a

stream of wastewater from the facilities. They described that these wastes yielded high methane

production because they contain high concentration of proteins and lipids.

The pretreatment methods they found to be optimal for this kind of waste was the

combined thermal and enzymatic pretreatment. The thermal pretreatment for these kinds of

waste consisted on heating the organic feed to a temperature of 120C for a heating time of 5

minutes. After the heating time commercial alkaline endopetidose were added at a rate of 2-10

g/l to the mixture. This yielded a 37-51% increase of the methane production of the mixture. The

results were an approximate methane yield of 0.57 m3 per kg of VSadded.

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Municipal Residential waste

Municipal waste is the most universal kind of waste found all around the world,

everywhere there is social activity involved, there will be enough municipal waste to be

considered into account. Activated sludge has been the most widely used biological material

used for municipal waste water treatment. The residual waste activated sludge (WAS) from the

mentioned process usually is used for further anaerobic processes. As mentioned before,

municipal wastewaters have a potential of using anaerobic processes to reduce the organic

materials, reduce odors and also use the biogas byproduct produce to obtained renewable energy

from it. Several approaches are being followed to include this kind of technology into developing

countries to enable them access to energy sources that uses their available resources without

depending on foreign oil.

Several pretreatments have been studied in order to make the municipal waste anaerobic

digestion even more efficient. These kinds of pretreatments include, thermal, chemical,

ultrasonic and thermo chemical pretreatments.

One of the most interesting studies I read from Kim et al 200313, they studied the effects

of various pretreatments methods to enhance waste activated sludge in anaerobic digestions.

They studied the thermal, chemical, ultrasonic and thermo chemical pretreatments found their

optimal parameters and concluded the best result pretreatment method.

They obtained that the optimal conditions for thermal pretreatment were a heating

temperature of 121C and a heating time of 30 minutes. When they performed the chemical

pretreatment the optimal conditions were adding 7 g per liter of NaOH.

The last optimal conditions found were when performing the ultrasound pretreatment to

the sludge. The optimal conditions were exposing the material to a frequency of 42 kHz for 120

minutes. The last pretreatment method was the thermo chemical one. They combined the optimal

parameters of the thermal and chemical (121C and a heating time of 30 minutes and adding 7 g

per liter of NaOH) to see the effects of the methane production.

The experiment showed that the thermo chemical pretreatment method was the one that

yielded better results of all. The results were an increase of methane production in approximately

35%. The biogas production and the methane production were 5037 l per m3 WAS and 3367 l

per m3 WAS, respectively.

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Water Hyacinth

Water hyacinth anaerobic digestion is very different from the cases studied before. Plant

materials are more difficult to biodegrade than animal manures24. The reason for this statement is

that the hydrolysis of cellulose material is a very slow process and is determine to be the rate

limiting step on the anaerobic digestion. That is the reason why there have been many attempts

of studying different pretreatment methods to reduce the effort needed to metabolize the

cellulose in plants.

A study performed by Ofoefule et al 2009, they compare four pretreatment methods for

water hyacinth on a 121 L digester. The pretreatment methods were: dried and chopped alone

(mechanical pretreatment), dried and treated with KOH (Alkali chemical pretreatment), dried

and combined with cow dung (co digestion).

The water hyacinth was anaerobic ally digested without any kind of pretreatment to

compare the results after the pretreatments were executed. The results from the fresh untreated

water hyacinth were the following: The mean biogas yield was 8.48±3.77 l per total mass. This

biogas yield was obtained after 9 days on the digester. The gas analysis showed that the biogas

was 65% methane and 34.94% CO2.

The results from the pretreatments were as follows: when it was dried and chopped alone

(mechanical pretreatment), the mean biogas yield was 9.75±3.40 l per total mass. This biogas

yield was obtained after 8 days on the digester. The gas analysis showed that the biogas was 60%

methane and 39.94% CO2. When it was dried and treated with KOH (alkali chemical

pretreatment), the mean biogas yield was 9.51±8.63 l per total mass. This biogas yield was

obtained after 5 days on the digester. The gas analysis showed that the biogas was 71% methane

and 28.94% CO2.

When it was dried and combined with cow dung (co digestion), the mean biogas yield

was 11.88±5.01 l per total mass. This biogas yield was obtained after 6 days on the digester. The

gas analysis showed that the biogas was 64% methane and 35.94% CO2.

On this study it can be concluded that the co digestion process was even more efficient that

any of the pretreatment methods used.

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VIII. Anaerobic digester plant energy from combustion.

Combustion is the number one means in which biogas can be taken advantage at most. On

this reaction methane combines with oxygen to release energy in the form of heat. The chemical

reaction for methane combustion is:

CH4 + 2 O2 � CO2 + 2 H2O.

On this reaction one molecule of methane (CH4) reacts with two molecules of oxygen to

release Carbon dioxide, water and energy. This energy can be calculation with the stoichiometric

reaction as follows:

CH4 + 2 O2 = CO2+ 2 H2O

-75 kJ/kg + 0 = -394 kJ/kg + 2(-242kJ/kg) + energy in the form of heat

Energy in the form of heat = 394 kJ/kg +2(242kJ/kg)-75kJ/kg

= 803 kJ/mol

In order to change this energy in the form of heat , now expressed in a molar basis into a

mass basis then we need to obtain the molecular weight of CH4 (0.016 kg/mol). This would

yield that there are approximately 50 MJ in every Kg of Methane.

Now based on this combustion reaction, the energy from anaerobic digestion can be

calculated for each different scenario of waste types. For example:

A hypothetic dairy farm that has 200 cows, and has a daily production of manure of approximate

700 kg. Assuming that this manure is reduce to 20 % solids and it is processed in an anaerobic

digester with the pretreatment methods explained in section VI, obtaining a biogas yield of 443

ml per g of dry matter.

Then each day the farm would produce 63,000 liters of biogas which equals 63 m3 of

biogas with an approximate methane content of 65%. Therefore the amount of energy obtained

from the combustion of the biogas would be 2,337 MJ of energy, assuming a methane density of

0.876 m3/kg. If this energy was to be produced into electricity in an internal combustion engine

with 30% efficiency, then the amount of electricity produce would be approximately 194 kWh

Based on this example, there are several possibilities of usage of the anaerobic digesters for

renewable energy production.

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IX. Conclusions

Activated sludge has been the most widely used biological material for industrial

wastewater treatment. But now I foresee that anaerobic digestion will become more important as

the need for renewable form of energy and water pollution removal systems increase.

From the information I have gathered from waste sources in Puerto Rico I can conclude that

here are the available waste resources to make anaerobic digestion a good and solid industry for

an island very depended to foreign oil.

But even today I have seen that Puerto Rico is in no way advance on the anaerobic digesters

implementations nor investigation. This gives a new option for researches and investors to

concentrate in Puerto Rico as an alternative for renewable energy sources.

Based on the data and information for this independent study I can conclude that the ideal

anaerobic digester system would be as one of the following:

-I would implement an anaerobic digestion system with cow dung as the feed waste with a

thermo chemical pretreatment before the hydrolysis step.

-or I would use the waste activated sludge that right now is being produced by existing

wastewater treatment plants and use it as feed waste for an anaerobic process also with a thermo

chemical pretreatment before the hydrolysis step.

Both scenarios could be implemented with a combine heat and power system that can

convert around 33% of the energy to power and the remaining 66% into heat making this a very

efficient energy recovery system.

These two scenarios would be the most cost effective implementation due to their biogas

yield and also the use of existing facilities.

X. References 1-Sheehan G J & Greenheld P F , Utilization treatment and disposal of distillery waste water 14 Water Res 14, 257 277 2-Costle D M 11979) Effect of distillery wastes on the marine Environment United States Environmental Protection Agency Washington DC

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