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Innovations for Long Term Resilience and Sustainable Nexus of
Food, Energy and Water System
Akrum H. Tamimi1
1
Shincci-USA: Executive Vice President-Engineering, Design and Technology Officer; and
Professor of Practice of Biosystems Engineering, The University of Arizona, Tucson, AZ, USA
*
Corresponding Author: akrumt@arizona.edu
Abstract
The majority of wastewater treatment facilities in the United States and around the world partially
treat the organic solids generated as a byproduct of the wastewater treatment process by anaerobic
digestion. Anaerobic digestion generates Class B biosolids which can contain millions of fecal
coliforms bacteria and other enteric pathogens per gram of solids. Anaerobic digestion requires
heating and keeping the organic solids above 35°C for at least 15 days as per U.S. EPA rule 503
requirements.
Approximately 7.1 Million dry tons of wastewater sludge are generated each year in the U.S. The
wastewater sludge is usually in the form of liquid at percent total solids of less than 10% (Greater
than 90% water). This amounts to more than 64.5 Million m3 per year of Class B sludge; of which,
15% are incinerated, 28% are landfilled and 36% are applied to agricultural land.
Huge cost and environmental problems are associated with this practice. The problems range from
the release of billions of enteric pathogens into the environment to the emitting of tons of
greenhouse gases into the atmosphere contributing to the climate change.
Tamimi. A.H., 2020
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It requires more than 750 KWH per wet ton of cake to dry sludge from 15% total solids to 90%
total solids. We have adopted and in the process of improving on a treatment technology that
dehumidifies the sludge to 90% solids with the consumption of 175 KWH per wet ton of sludge in
a pilot project. The new improved technology uses new thermodynamics principles and will
require less than 100 KWH per wet ton of sludge cake. We constructed a controlled environment
greenhouse adjacent to the sludge dehumidification pilot system. The dehumidification system
produces disinfected organic pellets that are used as rich energy and fertilizer sources. We are also
producing high quality condensate water, electric power, waste heat, and CO2. All those products
are being used in the green house to enhance production and yield of agricultural crops.
Background
The majority of wastewater treatment plants in the United States partially treat the organic solids
generated as a byproduct of the wastewater treatment process through using anaerobic digestion
processes (Figure 1). Treating wastewater sludge using anaerobic digestion process generates
Class B biosolids which can contain millions of fecal coliforms bacteria and other enteric
pathogens per gram of solids. Anaerobic digestion requires heating and keeping the organic solids
above 35°C for at least 15 days as per U.S. EPA rule 503 requirements (U.S. EPA, 1993). This
requires the construction, operation and maintenance of huge expensive reinforced concrete
digesters that generate odors and require air scrubbing at a huge cost to public utilities. Treating
and pumping water and wastewater is at the top of energy needs for municipalities across the
United States (Corum and Lovely, 2006). According to the California Energy Commission, 56%
of energy usage by municipalities is spent on water and wastewater treatment facilities. Energy
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efficiency in water and wastewater facilities would make huge savings benefiting water and
wastewater utilities and taxpayers.
Approximately 7.1 Million dry tons of wastewater sludge are generated each year at approximately
16,000 municipal wastewater treatment facilities in the U.S (Water Environment Federation,
2010). The wastewater sludge is usually in the form of liquid sludge with percent total solids less
than 10%. This amounts to 64.5 Million m3 per year; of which, (Figure 2) 15% are incinerated,
28% are landfilled and 36% are applied to agricultural land (Center for Sustainable Systems,
2015).
When the generated sludge is at a percent total solids less than 10%, that requires the hauling of
18.2 Million m3 of sludge to landfills and 23.1 Million m3 of sludge to land application sites on
yearly basis which amounts to millions of hauling and trucking kilometers.
Huge problems are associated with this practice: 1) the release of a minimum total of 1.3×1018
fecal coliforms bacteria and enteric pathogens per year into landfills; 2) the release of a minimum
total of 4.7×1018 fecal coliform bacteria and enteric pathogens per year into agricultural lands and
potentially to ground water; 3) emitting millions of tons of greenhouse gases, such as CO, CO2
and CH4, into the atmosphere due to releasing and/or flaring digester’s methane gas into the
atmosphere and from extensive hauling and trucking; and 4) the destruction of US highways and
bridges due to the repetitive loading and unloading by the heavy trucks and tankers hauling the
mostly water sludge to landfills.
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Objectives
1. Introduce a new transformative low cost technology for converting liquid organic waste
such as wastewater sludge to dry disinfected product that has no odor; and has high caloric
and nutrient contents. The process also produces reusable condensate water.
2. Demonstrate the reduction of hauling liquid organic waste through increasing percent total
solids to 90% (10% water content).
3. Demonstrate the use of the generated treated byproducts in energy generation and
agricultural production.
Methods and Materials
To demonstrate the new technology, a system design for a treatment plant is introduced here and
the technology is explained and demonstrated through the design.
Input Design Parameters
Influent rate to Wastewater Treatment Facility “A” is 15,520 m3 per day (4.1 Million Gallon Day).
The treatment facility is located in United States south west and it serves a community of 30,000
people. In 2015, the treatment facility generated 15,484 m3 of untreated stabilized liquid sludge at
an average percent total solids of 3.46% per year (50 m3 per day based on 6 days per week
operation). the liquid sludge was hauled 48 Kilometers for treatment to class B level as per U.S.
EPA Rule 503 (U.S. EPA, 1993) and was reused for land application. The dry solids present in the
liquid sludge can be calculated as 537 tons per year or 1.7 tons per day based on 6 days per week
operation. The hauling and treatment cost totaled $383,536 for year 2015 which represents a cost
of $0.0248 per liter of liquid sludge.
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Description of the New Technology
The new sludge treatment system consists of two integrated modules: the first is a spiral filter
dewatering system that dewaters liquid sludge from 3.46% to 20.0% total solids; and the second
is a heat pump Dehumidification module that dries the 20.0% dewatered cake coming out of the
spiral dewatering system to 90.0% total solids resulting in Class A biosolids that meets vector
attraction reduction as per EPA 503 rule (U.S. EPA, 1993). Figure 3 shows a schematic of the new
sludge treatment system.
Spiral Filter Dewatering System
The Spiral filter dewatering system can dewater sludge from 3.46% to 20.0% total solids with less
than 1 KWH per dry ton of sludge, it also requires about 5 Kg of polymer per dry ton of solids.
The thickened sludge caked is then dropped into the Dehumidification module for drying.
Low Heat Dehumidification Heat Pump
The dehumidification heat pump installed in the new sludge treatment system utilizes the
refrigeration principal to cool and dehumidify hot wet air. Through the heat pump principal, the
heat pump recycles the latent heat released from steam congealing to water liquid. A
Dehumidification heat pump is equal to the Dehumidification process (moisture removal or
moisture dehumidifying) plus a heat pump process (energy recycling). A Dehumidification heat
pump can internally collect all the latent heat and sensible heat during air exhaust, bringing no
waste heat to the outside.
The evaporation of sludge moisture absorbs latent heat; and the condensation of the generated
vapor on the heat pump cycle releases latent heat. The evaporation process absorbs the same
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quantity of latent heat that the condensation process produces, according to the laws of
thermodynamics and the law of conservation of energy. As a result, the drying process does not
require additional heat capacity, resulting in the reduction of energy costs. The energy consumed
during the process is only the electricity needed to operate the compressors and the fans or air
handlers in the dehumidification system. The new system adopts a belt type closed cabinet
dehumidification drying system. There is no need for a treatment system for odor or gas buildup.
The sludge cake is shaped as thin spaghetti before it is placed on the mesh belt increasing the
sludge surface area to enhance the evaporation of the water entrapped in the wet sludge cake.
The dry air coming into the system shown in Figure 4 starts at 80°C, the temperature is reduced as
the air is moving upward and as its moisture content increases the temperature decreases to reach
a temperature of 50°C at the top of the drying module where it enters the dehumidification heat
pump. Sludge cake stays on the two drying mesh belts for a total period of two hours.
Mesh Belts
Mesh Belts are used in the new sludge dehumidification system with a variable frequency control
to vary the Mesh Belt speed. The speed of the Mesh Belt is adjustable to enable the adjustment of
the output dried biosolids. The speed can be reduced to arrive at drier biosolids or it can be
increased to arrive at wetter biosolids.
During the sludge dehumidification process the dehumidification heat pump dries by means of hot
air the wet sludge killing all viral, bacterial and parasite pathogens, hence producing Class A
biosolids that has no odor with all nutrient and caloric contents intact. The hot air and the
condensate water are captured within the system.
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Energy Requirement
To dewater 50 tons of sludge per day at Wastewater Treatment Facility “A” from 3.46% to 20%
total solids, the spiral filter dewatering system requires one Kilo Watt Hour (KWH) per dry ton of
sludge. The 50 tons of liquid sludge at 3.46% total solids contain 1.73 tons of dry solids. Therefore,
the spiral filter dewatering system requires about two KWH per day.
When the 50 tons of sludge per day are dewatered from 3.46% to 20% total solids, their weight is
reduced to 8.6 tons of untreated sludge cake per day. The dehumidification system requires an
average of 175 KWH to dehumidify one ton of sludge cake from 20% to 90% total solids.
Therefore, to dehumidify 8.6 tons of sludge cake per day from 20% to 90% total solids the
dehumidification system requires 1,505 KWH per day.
Results
To treat the 50 m3 of liquid sludge generated at Wastewater Treatment Facility “A” per day using
this patented efficient dewatering and dehumidification treatment system, the 50 m3 (equivalent to
50 Tons) per day are reduced to 1.9 tons of Class A biosolids at 90% total solids. Figure 5 shows
the drastic reduction in the weight of sludge after dewatering and dehumidification as a function
of percent total solids. For Wastewater Treatment Facility “A”, a reduction of sludge weight goes
from 15,484 tons per year to 595 tons per year representing a weight reduction factor of 26 (2,600%
reduction).
In addition, an estimated volume of 5,389 liters of condensate fresh water at ambient temperature
are generated every day.
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The 2015 cost to haul and treat the liquid sludge generated at Wastewater Treatment Facility “A”
was calculated earlier to be $0.0248 per liter of liquid sludge. Capital and financing costs,
operational, maintenance and electric cost of the new system over 3 years with municipal interest
rate of 5.0% per year results in a $0.0232 per liter of liquid sludge with a saving of $0.0016 per
liter for the first three years. This represents a total savings of $24,775 for the first three years.
For years 4 through 15, the cost of drying, dehumidification, electrical cost based on $0.08 per
KWH, operational cost and maintenance cost will be in the order of $0.011 per liter of liquid
sludge. This represents a yearly saving of $214,834 over the current practices used in Wastewater
Treatment Facility “A”. The savings over 11 years (years 4 through 15) represents a total of
$2,363,174 assuming 15 years as the operational life of the dewatering and dehumidification
system.
Conclusions
The food, energy and water nexus system is comprised of a Genset to generate electricity, waste
heat and CO2 gas; a controlled environment green house and a dewatering-dehumidification
modules. All are installed in a wastewater treatment facility that produces preferably undigested
stabilized liquid sludge.
Undigested stabilized liquid sludge is treated to Class A fertilizer and energy pellets using the
dewatering-dehumidification treatment system. The energy pellets are used to generate energy to
run the Genset. The Genset generates electricity and waste heat that can be used to run the
dewatering-dehumidification system. The condensate fresh water can be used to irrigate
agricultural crops in the controlled environment greenhouse. The generated electricity can also be
used to light the controlled environment green house at night and for cooling and heating the green
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house. The CO2 gas generated from the Genset can be pumped into the controlled environment
greenhouse to provide more efficient agricultural growth and yield.
The tangible savings in implementing a food, energy and water nexus system are huge. Those
savings can be reflected by the drastic reduction of the high cost of treating and hauling sludge at
wastewater utilities.
The intangible savings are reflected in eliminating the release of greenhouse gases into the
atmosphere, the reduction of contaminating agricultural soils and potentially ground water, saving
roads, bridges and highways from repetitive loading and unloading of heavy trucks and tankers,
and improving public health.
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References
Center for Sustainable Systems, University of Michigan. 2015. “U.S. Wastewater Treatment
Factsheet.” Pub. No. CSS04-14.
Corum L. and Lovely L. Controlling Municipal Energy Cost. Distributed Energy, NovemberDecember 2006. Pp.16-26.
U.S. EPA. 1993. Federal Register: February 19, 1993. 40 CFR Part 503; as amended at 64 FR
42571, Aug. 4, 1999.
Water Environment Federation. 2010. Land Application and Composting of Biosolids: Q&A/Fact
Sheet.
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Figures
Figure 1: Wastewater Treatment Process
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Figure 2: Uses of Wastewater Sludge in the US
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Figure 3: Schematic of the Spiral Filter Press Dewatering Module and the Dehumidification
Module in the New Sludge Treatment Technology
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Figure 4: Process Flow of the Dehumidification Drying Module
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Figure 5: Weight Reductions in Sludge as a Function of Percent Total Solids
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