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1 INTRODUCTION
1.1 Motivation
Anaerobic digestion is the most widely used of the processes in wastewater treatment
plants. It treats the solids generated on the primary treatment process, converting them
to a stable product for use or disposal. Its purpose is to reduce pathogens, reduce
biomass quantity by partial destruction of volatile solids, and produce usable gas as
byproduct. It is really important to take a deeper dive into the studies of these
installations with the aim of improving working conditions and design without never
forget the impact of the equipment on the environment. The use of mathematical models
is not strictly necessary, but they allow us to study a big quantity of possible alternatives
and situations. Thus, it is possible to have different designs before the construction of
the digester, and definitely, an improvement of the resource.
However, the flow and the mixing processes in these digesters are often evaluated with
experimental expensive studies or with hard analytical techniques. In addition, the
analytical methods that usually are utilized are insufficient to have a correct and
complete characterization of mixing and transport processes. On the other hand, in the
last years there is a great improvement of numerical methods applied in this topic.
Thanks to velocity and computational capacity increment, methods like “computational
fluid dynamic” (CFD) allow us to develop simulation of complex hydraulic domains. In
this way, this numerical method is presented today as a powerful tool, complementing
advantageously experimental and analytical studies.
This project is part of a research and development agreement between the “Universidad
Politécnica de Valencia” and the “Grupo Aguas de Valencia”, a company for which is
conducted the study. In this work is developed the numerical part, through CFD models,
to estimate a possible improvement of the working conditions of an operative anaerobic
digester in Ontinyent – Valencia. The research has been conducted with the
collaboration of the D.ra P. Amparo López Jiménez of the “Departamento de Ingeniería
Hidráulica y Medio Ambiente” in the Universidad Politecnica de Valencia.
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1.2 Justification
Aguas de Valencia was set up in 1890 under the original name of “Sociedad de Aguas
Potables y Mejoras de Valencia”, S. A., changing its name to the current one in June
1988. We started life as the company contracted by the Valencia City Council to
modernize the drinking water supply to the city by building settling pools, filters and
deposit tanks alongside the river Turia.
Since then, the Group manages all aspects of captation, treatment and water distribution
in the city of Valencia and in most of the populations of the metropolitan area. It also
administers the Metropolitan Water Supply System which provides drinking water to
the municipalities of different regions of Valencia. To do this, two water treatment
stations are operating; they are supplied by the superficial water taken from rivers Júcar
and Turia-Manises Dam and Picassent RealOne.
Since then, the Group manages all aspects related to drinking water sourcing, treatment
and distribution to the city of Valencia, as web as to most of the towns and villages
within its metropolitan area. Moreover, we also administer the Metropolitan Supply
System which distributes water to a variety of municipalities in the Valencia area. In
order to do this, we operate potabilizing plants supplying surface water coming from the
Jucar and Turia rivers –La Presa in Manises and El Realón in Picassent.
Throughout its history, to achieve the development of the activities and especially its
expansion in the different regions, this organization has been creating or participating in
various companies in different geographic areas. Thus, at present, it carries out its
management in eight Spanish regions: Aragon, Cantabria, Catalonia, Valencia,
Extremadura, Murcia, Navarra and the País Vasco.
In the nineties, the company worked with various international institutions to spread
their know-how in different Latin American countries as Venezuela, Costa Rica,
Colombia, etc.
Regarding his future, Aguas de Valencia Group aims its imminent implementation
throughout the national territory, in the investigation of new enhancements to
incorporate into their plants and the development of R + D + I (Research, Development
and Innovation).
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1.3 State of the art
1.3.1 General description of Anaerobic digesters
Anaerobic digestion occurs as the result of a complicated set of chemical and
biochemical reactions. Reactions occur within the context of a complex ecosystem
involving many types of bacteria, with each type providing a unique and indispensable
bio-transformation. Figure 1.1 shows a simplified representation of the anaerobic
digestion process, which is divided into the following three stages: hydrolysis,
formation of soluble organic compounds and of short-chained organic acids, and
methane formation.
Extracellular
Enzymes
Acid
Producers
Methanogens
Complex
Organics
Soluble
Organics
Organic
Acids
Methane,
CO
2
Hydrolysis
Carbohydrates,
Proteins,
Lipids,
Phosphorylated
Organics
Glucose,
Amino Acids,
Fatty Acids,
PO
4
-3
Acetic,
Propionic,
Lactic + Cells
Cells,
Stabilized
Organics
Fig. 1 .1 – Microbiological pathway of anaerobic digestion
In the first stage (hydrolysis), the proteins, cellulose, lipids, and other complex organics
are made soluble. In the second stage (acid formation), the products of the first stage are
converted to complex soluble organic compounds including long-chained fatty acids;
these soluble organic compounds are then broken down to short-chained organic acids
(know as acidification). In the third stage (methane formation, or methanogenesis), the
organic acids are converted to methane and carbon dioxide. The overall extent of
stabilization by anaerobic digestion is measured by the amount of volatile solids
destruction that occurs through the digester. Because anaerobic digestion is biologically
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mediated and depends on the growth of microorganisms, complete volatile solids
destruction does not occur, with 40 to 65% being typical. Lower percentage destruction
occurs with biological solids or solids containing significant concentrations of materials
that are difficult to degrade and with a low mixing in the digester.
Of the many environmental factors that affect the rates of the three anaerobic digestion
reactions, the most important are the solid retention time, effectiveness of mixing,
hydraulic retention time, temperature, pH, and the presence of toxic materials.
Uniformity of environmental conditions within the anaerobic digester is critical and
largely determines the maximum possible rate of digestion and the potential for digester
upsets. The degree of mixing determines the uniformity of conditions in the digester.
Solids and Hydraulic Retention Times: The sizing of anaerobic digester is
based on providing sufficient retention time in these well-mixed reactors to allow
significant volatile solids destruction to occur. Sizing criteria, expressed either as solids
retention time (days, calculated as the mass of solids in digester divided by the mass of
solids removed per day), and the hydraulic retention time (days, the working volume
divided by the volume sludge fed to the digester per day). If there is no change in solids
concentration within the digester, the hydraulic retention time is the same as the solids
retention time. The solid and hydraulic retention times and the extent of each of the
three reactions occurring during anaerobic digestion are directly related: an increase in
solids retention time increases the extent of each reaction; a decrease in solids retention
time decreases the extent of reaction. There is a minimum critical retention time for
each reaction and if this is not provided, the bacteria cannot grow rapidly enough to
remain in the digester, the reaction mediated by those bacteria will cease, and the
digestion process will fail.
Temperature: Temperature is important in determining the rate of digestion,
particularly rates of hydrolysis and methane formation. From a design standpoint, both
the design operating temperature and the ability to maintain that temperature within
close tolerances are important. The design operating temperature establishes the
minimum solids retention time required to achieve a given amount of volatile solids
destruction. Most anaerobic digestion systems are designed to operate in the mesophilic
temperature range, approximately 35 °C. Some systems have been designed to operate
in the thermophilic temperature range, approximately 55 °C. Advantages claimed for
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thermophilic digestion include improved de-waterability, increased pathogen
destruction, and increased scum digestion. While selection of design operating
temperature is important, maintaining a stable operating temperature in the digester is
more important. This is because bacteria involved are sensitive to temperature changes.
Temperature changes greater than 1 °C/d can result in process failure.
pH: Anaerobic bacteria, particularly the methane formers, are also sensitive to pH.
Optimum methane production occurs when the pH level is maintained between pH 6.8
and 7.2. Acid formation continuously occurs during the digestion process and thends to
lower the digester pH. However, methane formation also produces alkalinity, primarily
in the forms of carbon dioxide and ammonia. These materials buffer changes in pH by
combining with hydrogen ions. A reduction in digester pH inhibits methane formation.
Mixing, heating, and feed-system designs are important in minimizing the potential for
this type of process upset. Provisions for the addition of chemicals, such as lime,
sodium bicarbonate, or sodium carbonate, to neutralize excess acid in an upset digester
should be included.
Toxic Materials: If concentrations of certain materials, such as ammonia, heavy
metals, light metals cations, and sulfide, sufficiently increase in anaerobic digesters,
they can create unstable conditions within the digester. Toxic conditions can occur as a
result of a sudden change in digester operation, such as overfeeding or excessive
addition of chemicals, or as a result of a shock loading of these materials in the plant
influent. The most common effect of excess concentration of these materials in the
digester is inhibition of methane formation. This leads to volatile acid accumulation, pH
depression, and digester upset, as previously discussed.
Applicability: Anaerobic digestion may be considered beneficial for stabilization
when the volatile solids content is 50% or higher and if no biologically inhibitory
substances are present or expected. Digestion of primary solids results in better solids-
liquid separation characteristics than activated sludge. Combining components will
results in settling characteristics that are better than activated sludge but not as good as
primary sludge alone. Chemical residuals containing lime, alum, iron, and other
substances can be successfully digested if the volatile solids content remains high
enough to support the biochemical reactions and no toxic compounds are present.
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The advantages of anaerobic digestion include the following:
Energy is produced. Methane is produced and can be used to heat and mix the
reactor.
The quantity of total solids for disposal is reduced, approximately 30% to 40%
may be destroyed. The volatile solids are converted in methane, carbon dioxide
and water. Approximately 40 to 60% may be destroyed.
The product is stabilized solids that can be used for land application.
Pathogens are destroyed to a high degree during the process.
In municipal treatment facilities, most organic substances are readily digestible,
except for lignins, tannins, rubber and plastics.
The disadvantages associated with anaerobic digestion included the following:
The digester is easily upset by unusual conditions and erratic or high loading,
and slow to recover.
The process requires high operational control.
Heating and mixing equipment are required for satisfactory performance,
making this process equipment-intensive.
Large reactors are required because of the slow growth of methanogens and
required solid retention times of 15 to 20 days for a high-rate system. The capital
costs are high.
The resultant supernatant sidestream is a strong waste stream that adds to the
loading of the wastewater treatment plant. It contains high concentration of
biochemical oxygen demand (BOD), chemical oxygen demand (COD),
suspended solids (SS), phosphorous and ammonia-nitrogen.
Cleaning operations are difficult and dangerous because of the closed vessel.
The possibility of explosion exists as a result of inadequate operation and
maintenance, leaks, or operator carelessness.
1.3.2 Process Variations
Three process configurations for anaerobic digestion in common use are low-rate, high-
rate and two-stage digestion. In addition, anaerobic digestion can also be operated in
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two temperature regimes: mesophilic (30 to 38°C) and thermophilic (50 to
60°C),(Verma, 2002).
Low-Rate digestion: Low-rate digesters are the oldest anaerobic stabilization
systems, Figure 1.2.
Fig. 1.2 – Low-rate anaerobic digestion
The digester consists of a cylindrically shaped tank with a sloping bottom and a flat or
domed roof. No mixing is provided in this system. Because of it, stabilization in this
kind of digester results in a stratified condition within the digester. Methane gas
accumulates in the headspace of the tank and is draw-off for storage or use. Scum
accumulates on the liquid or supernatant surface. The supernatant is drawn off and
recycled either to the primary clarifier or to the secondary treatment process. The
supernatant contains high ammonia and phosphorous concentrations. The stabilized
solids settle to the tank bottom for removal and further processing. Low-rate digestion is
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characterized by intermittent feeding, low organic loading rates, no mixing and other
than that caused by rising gas bubbles, large tank size because of the small effective
volume, and detention times of 30 to 60 days. Grit and scum layers will accumulate on
the bottom and top of the tank, respectively, thereby decreasing the effective volume.
An external heat source maybe or may not be present to increase the digestion rate.
Conditions are not maintained for optimum digestion. This type of digestion has
traditionally been considered only for small plants, less than approximately 4000m
3
d.
Low-rate digesters are seldom built today.
High-Rate Digestion: High-rate digesters are characterized by supplemental
heating and mixing, uniform feeding rates, and sludge thickening before digestion,
Fig.1.3.
Fig. 1.3 – High-rate anaerobic digestion
In properly designed and operated digesters, these factors result in uniform conditions
throughout most of the digester. As a result, the tank volume required for adequate
digestion is reduced and the stability of the process is improved. Heating the sludge
digestion increases the microorganism growth rate, the digestion rate and gas
production. High-rate anaerobic digesters may offer several advantages over mesophilic
digestion, including increased reaction rates that can result in smaller digester volumes,
increased destruction of pathogens and better dewatering characteristics. Limitations of
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the process include extreme sensitivities of the organism to the defined temperature
range, a higher net energy input compared with the mesophilic process, and the
production of digested sludge with more offensive odor. Several heating methods have
been used for anaerobic digesters, including steam injection, internal heat exchangers
and external heat exchangers. External heat exchangers are the most popular because of
their flexibility and ease of maintaining the heating surfaces. Internal coils can foul
because of caking and, as a result, will have to be removed, or the digester will have to
be emptied to clean them. Hot water supply temperatures to heat exchanger surfaces are
maintained between approximately 50 and 62 °C. At higher temperatures than 62 °C,
caking on the heat exchanger surfaces becomes more likely. Auxiliary mixing of the
digester contents reduces thermal stratification, dispersing the substrate for better
contact with the active biomass and reducing scum buildup. Mixing also dilutes any
inhibitory substances or adverse pH and temperature feed characteristics, thereby
increasing the effective volume of the reactor. Continuous or regular intermittent
feeding is beneficial to digester operation because it helps maintain steady-state
conditions within digester. The methanogens are sensitive to changes in substrate levels.
Uniform feeding and multiple feed-point locations in the tank can alleviate or reduce
shock loading to those microorganisms. Excessive hydraulic loading should be avoided
because it decreases detention time, dilutes the alkalinity necessary for buffering
capacity and requires additional heat energy
Two-Stage Anaerobic Digestion: Two-stage digestion is an expansion of the
high-rate digestion technology that divides the functions of fermentation and solids-
liquids separation in two separate tanks in series, Fig.1. 4.
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Fig. 1.4 – Two-stage, high-rate anaerobic digestion
The first tank is a high-rate stabilization system, while the second is for solids-liquid
separation. The second reactor does not have mixing or heating facilities unless it is also
used to provide standby digester capacity. It may serve several other functions such as
providing storage capacity and insurance against short circuiting of the process.
Anaerobically digested solids may not settle well, resulting in a supernatant containing a
high concentration of suspended solids that can be detrimental to the liquid wastewater
treatment system when re-circulated. Several reasons for poor settling characteristic
include incomplete digestion in the primary digester (which generates gases in the
secondary digester and causes floating solids) and fine-sized solids that have poor
settling characteristics. This latter case is associated with secondary or tertiary solids,
including chemical solids. Two-stage digesters using both thermophilic digestion
followed by mesophilic digestion are in developmental stage and may prove to have
superior operational characteristics
Design Parameters
Design of anaerobic digesters has been based on solids retention time, organic loading
rate (volatile suspended solids per volume) and volume per capita. Design parameters
for low- and high-rate digesters are detailed in the Table 1.1.
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Tab. 1.1 – Typical design parameters for low-rate and high rate digesters
For estimating feed volumes at domestic plants in the absence of operating data, the
volume figures per capita can be used. Low-rate digesters are organically loaded at rates
equal to approximately 0.5 to 1.5 kg/m
3
volatile suspended solids (VVS) ·d. High rate
digesters with mixing and heating are characterized by organic loading rates of 2 to 3
kg/m
3
VVS·d. Solids retention times are approximately 30 to 60 days for low-rate
digestion and 15 to 20 days for high rate digestion at mesophilic temperatures.. For two-
stage digestion, the solids retention time is calculated only for the first reactor in the
system because the second tank is used mostly for solids settling and storage, as well as
gas storage.
Volatile Solids Loading and Destruction: Volatile solids loading refers to the
mass of volatile solids (VS) added to the digester each day divided by the working
volume of that digester. (kg VS/m
3
·d). Loading criteria typically are based on sustained
loading conditions (peak month or peak week solids production), with provisions for
avoiding excessive loading during shorter time periods. A typical design sustained peak
volatile solids loading rate is 1.9 to 2.5 kg VS/m
3
·d. The upper limit is determined by
the rate of accumulation of toxic materials, particularly ammonia, or washout of
methane formers. A limiting value of 3.2 kg VS/m
3
·d is often used.
Gas production: The specific gas production at municipal plants can be estimated
by using the relationship of approximately 0.8 to 1.1 m
3
/kg of VS destroyed. The
greater the percentage of fats and grease, the higher the expected specific gas production
as long as adequate solids retention time and mixing are provided because these
materials are the slowest to metabolize. The gas produced is collected either for use or
burning to avoid odor. As it rises through the sludge, gas is collected above the
Parameter Low rate High rate
Solids retention time,days 30-60 15-20
Volatile suspended solids loading, kg/m
3
·d 0,64-1,6 1,6-3,2
Volume criteria, m
3
/cap
Primary sludge 0,06-0,08 0,03-0,06
Primary sludge + trickling filter sludge 0,11-0,14 0,07-0,09
Primary sludge + waste activated sludge 0,11-0,17 0,07-0,11
Combined primary + waste biological sludge feed concentration,% solids-dry basis 2,0-4,0 4,0-6,0
Anticipated digester underflow concentration, % solids-dry basis 4,0-6,0 4,0-6,0
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digester‟s liquid surface and is released. The digester gas is then piped to the heating or
power equipment for immediate use, a gas holder for later use, or a waste-gas burner.
Mixing: Most manufacturers of digester mixing equipment will suggest the
appropriate type, size and power level of mixing equipment depending on the digester
volume and geometry. These suggestions are based on in-house studies and successful
experiences of other similar installations. Anaerobic digesters can be mixed with gas,
mechanical or pumped mixing systems.
Gas-injection systems using draft tubes are a common type of digester mixing system
in use today. They can provide enough mixing to ensure a completely mixed process. A
draft-tube gas-recirculation system consists of a series of large-diameter tubes into
which digester gas is released, causing the biomass to rise and mix as it approaches the
liquid surface of the tank. The number of draft tubes in a digester depends on its
dimensions. More than one draft tube is provided for digester with diameters greater
than 18m. Compressed gas is released inside the draft tube through top-entering lances
or laterally through the draft-tube wall near the bottom of the unit. The draft tube can be
installed on the roof or mounted on the digester floor using supports. The system is
supported by the required compressors and controls. A heating jacket can be installed
around the draft tube to provide mixing and heating.
Mechanical stirring system use rotating impellers to mix the digester contents. The
mixers can be either low-speed turbines or high-speed propeller mixers installed in draft
tubes. The draft tubes can be either internally or externally mounted on the digester. The
flow pattern for mechanically stirred and pumped mixing systems is from top to bottom.
In pumped mixing systems, externally mounted mixing pumps withdraw biomass from
the top center of the tank and re-inject it through nozzles tangentially mounted at the
bottom of the tank. Scum breaker nozzles are also provided at the liquid surface for
intermittent use in breaking up scum accumulations.
Heating: Maintaining a constant temperature in the digester improves the working
conditions of the process. Rapid changes in temperature can lead to process upset.
Controlling the temperature near an optimum value maximizes the rate of digestion,
thereby minimizing the requiring digester volume. To maintain digester temperature at a
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constant value and near optimum, heat is added to raise the temperature of incoming
sludge and offset the heat loss from the digester to the surrounding ground and air.
1.3.3 Main problems
Recently anaerobic digestion processes have taken much force, for the excellent results
obtained by the production of a very stable sludge, but it is still an ongoing study
looking for improvements because it is a sensitive system which can be seen disturbed
and not properly working.
One of the important reasons for failure is improper mixing provided in the digester.
Proper mixing is needed for optimal working conditions and operation since mixing or
agitation is required to homogenize the contents of digesters, to ensure uniform
distribution of substrate and microorganisms culture, to avoid settling of the heavy solid
particles to the bottom, to avoid flotation of biomass at the surface of the sludge, and to
maintain the desired pH and temperature of the sludge fir the microbial processes.
The working conditions of anaerobic digesters are affected primarily by the retention
time of digestible sludge (substrate) in the digester and the degree of contact between
incoming substrate and a viable bacterial population. These parameters are functions of
the hydraulic regime (mixing) in the reactors. Mixing in the digester is required to
distribute organisms, substrate, and nutrients uniformly, to transfer heat, and to maintain
uniform pH. Thus, mixing is regarded as essential in Anaerobic Digesters (Meynell,
1976; Sawyer and Grumbling, 1960). Furthermore, mixing aids in particle size
reduction as digestion progresses and in the removal of gas from the mixture. Mixing is
also required to prevent stratification and scum formation. In short, adequate mixing
provides a uniform environment, one of the keys to have a good digestion (Parkin and
Owen, 1986).
Inefficient mixing decreases effective system volume, which reduces the sludge
retention time and pushes the system towards failure. Studies with full-scale digesters
have shown that inefficient mixing may reduce the effective volume of the digester by
as much as 70%, leaving an actual volume utilization of only 30% (Monteith and
Stephenson, 1981). Parkin and Owen (1986) illustrated the effect of the SRT on digester
working conditions and proved that inefficient mixing causes digester failure. From a