Solar Dryers for Biosolids – Features and Design Information

Solar Dryers for Biosolids – Features and Design Information
Solar Dryers for Biosolids
– Features and Design Information
Huber Technology Inc.
Why biosolids drying?
Disposal of biosolids from wastewater is expensive. Looking beyond personnel and power
costs, biosolids disposal costs are the major operating costs for wastewater treatment plants.
Disposal costs are priced by weight. Mechanically dewatered biosolids typically contain 70 to
80 % water. By applying drying methods, additional water can be removed. This will result in
a significant savings in the haul-off costs.
By neutralizing pathogens in biosolids (Class A) land application of biosolids is desirable for
nutrient recycling. However with weather being a factor, land application is only feasible
during certain periods during the year. As a result large storage capacity would be needed to
hold the material until it is possible to resume land application under more favorable
Biosolids which are not land applied must be disposed on a landfill or incinerated. All options
discussed often require expensive long distance hauling.
Incineration is an option. The process involves evaporation of the contained water by means
of burning. The more water the biosolids contain (as in the case of mechanically dewatered
sludge), the more severe the effect on energy balance during incineration. Often some other
fuel needs to be added. Incineration fees are based on the bulk mass and its caloric value.
The wetter the biosolids are, the bigger the mass, resulting in more expensive incineration
Tipping fees are also based on the bulk mass and sludge consistency. Most landfill operators
will not accept semi-liquid slurries. The higher the water content of the sludge, the bigger the
mass which results in higher tipping fees.
Good mechanical dewatering of the sludge is required for all disposal options (with the
exception of land application in liquid form). However, even after excellent mechanical
dewatering, sludge from wastewater treatment still contain 70 to 80 % water.
Drying is used to remove most of the remaining water. The dried biosolids mass is only a
small fraction. Hauling and discharge cost are thus minimized through drying.
A wastewater treatment plant for a total population of 50,000 using an extended aeration
process for aerobic sludge stabilization produces about 1,378 short tons (1,250 metric tons)
of solids per year. If the dewatering characteristics of the waste activated sludge are
excellent, a dry solids ratio (DSR) of 25 % DS in the cake could be achieved. The dewatered
biosolids mass is 1,378/0.25 = 5,512 short tons (1,250/0.25 = 5,000 metric tons) per year.
If the dewatered biosolids are dried to a dry solids ratio of e.g. 75 % their mass is further
reduced to 1,378 / 0,75 = 1,837 short tons (1,250 / 0,75 = 1,670 metric tons) per year. This is
only about one third. The following graph illustrates how strong the bulk mass reduction is.
Abnahm e der Masse durch Trocknung
mass reduction through
ratio [%][%]
W asser
Relative mass of biosolids depending on their water content
If we assume that a big tractor trailer can haul a weight of 33 short tons (30 metric tons), 167
hauls per year are needed for the dewatered biosolids. Only 56 hauls are needed for the
dried product.
Number of yearly truck hauls for the example
The dried product is easy to handle and to store. If the product contains less then25 %
water there is not enough water for further biological degradation. The dried product should
not not emit odor.
Why solar drying?
Drying is a thermal process. Evaporation of water requires energy (evaporation enthalpy).
Evaporation of a lb of water at 68°F requires 1,055 Btu (of 1 kg at 20°C requires 0.68 kWh).
The evaporated water is turned into air moisture (humidity). Drying of biosolids at such a low
temperature generates little odor because volatile organic substances remain mostly in the
Solar radiation is readily available free energy which can be used for sludge drying.
Harvesting the sun’s energy for biosolids drying
The electric power consumption of solar dryers is 68,243 Btu 92,810 Btu/short ton (20 to 30
kWh/metric ton) of water evaporation. This is only 2 % to 3 % of the thermal energy needed
for water evaporation.
As of 2015 there are about 400 solar biosolids dryers installed at wastewater treatment
plants worldwide. The number is rising steadily.
Where is solar drying feasible?
Solar drying is a slow process requiring time and a large footprint. The process is particularly
useful for small and medium sized wastewater treatment plants where space is available.
Solar drying depends on the average sunshine intensity and other climatic conditions (e.g.
average monthly air temperature and humidity, i.e. the ambient air’s capacity to take up
vapor, i.e. its drying capacity).
Map for the average yearly sun irradiation energy in the US
Our example wastewater treatment plant serving a total population of 50,000 and producing
5,511 short tons (5,000 metric tons) of dewatered biosolids per year, requires evaporation of
5,111 – 1,837 = 3,274 short tons (5,000 – 1,670 = 3,330 metric tons) of water per year. In
practice irradiation energy of 1,548 to 1,857 Btu (1.0 to 1.2 kWh) is needed for the
evaporation of 1 lb (1 kg) water depending on climatic conditions. For our example 12,488
MM Btu (3,660,000 kWh) irradiation energy is needed per year. If the plant is located in
Missouri where the average yearly irradiation is about 507,200 Btu/(ft²*y) (1,600 kWh/(m²*y)
and a footprint of 12,488 MM Btu per y / 507,200 Btu / per ft² and per y = 24,621 ft² or 0.565
acres (3,660,000 kg/a / 1,600 kWh/(m²·a) = 2,288 m²) is needed.
Considering that the sun irradiation energy varies from year to year and that heat losses
must be taken into account, a footprint of about 0.70 acres (2,833 m²) should be selected.
The climatic conditions depend on the season. During summer the sun irradiation is strong
and long, the average ambient air temperature is high and the average relative humidity low.
During winter it is the opposite. The drying capacity of a solar dryer is thus high during
summer and low or even zero during winter. The challenge is wastewater treatment plants
generate biosolids throughout the year.
For this reason it is generally necessary to store liquid sludge or dewatered biosolids from fall
to spring. This requires a sufficiently large enough storage capacity.
How do solar dryers work?
If we hang our laundry on a washing line we do not need to supply heat. At 50°F (10°C) and
atmospheric pressure, air is water saturated if it contains 53.35 grains vapor/lb of dry air (7.9
g vapor/kg of dry air ). At 122°F (50 °C) it can carry 550.47 grains vapor/lb of dry air (89.2 g
vapor/kg of dry air) . Hot air can carry far more vapor than cool air. Warmer air has the same
absolute humidity, but a lower relative humidity, so it can take up more water.
If the air is not water saturated, i.e. if its relative humidity is below 100 %, it can take up more
water until it becomes water saturated. Our laundry will not dry on a foggy day when the air is
already water-saturated(relative humidity is 100 %). But it dries fast on a hot and sunny day.
Our wet laundry is gradually dried as liquid water evaporates into the air. The air and our
laundry are simultaneously cooled. This phenomenon is described as evaporation enthalpy.
Drying our laundry on a washing line is cost free, but it takes more or less time, depending
on the weather. It is fast on a hot and dry day, particularly if there is some wind blowing. But
the process is slow when the sun does not shine and the air is cold.
Solar biosolids dryers are based on the same principles as our laundry line.
A solar dryer is a greenhouse. When the sun shines through the glass, the air in the
greenhouse is heated by sun irradiation.
If a lb (kg) of water saturated ambient air at 50°F (10 °C) enters a greenhouse where it is
heated by sun irradiation to 122°F (50 °C), it can theoretically take up (603.8 – 53.4 grains of
vapor)/lb of dry air = 550.4 grains of vapor/lb of dry air (89.2 – 7.9 = 81.3 g vapor/kg of dry
air) until it is water saturated and returned to the environment. Most of the sun’s energy is not
used for the heating of the air, but as evaporation enthalpy. The air in the greenhouse is
simultaneously heated through sun irradiation and cooled by transmitting evaporation
enthalpy to the water. This is an equilibrium.
The sun supplies the required evaporation enthalpy for the drying process free of charge.
Solar drying even occurs during a cold but sunny winter day. However, winter days are short
in the northern hemisphere, so during winter solar dryers have a relatively low drying
Water evaporation in the greenhouse would cease as soon as the air became water
saturated. Ventilators and louvers are used to remove some warm and humid air from the
greenhouse and to replace it with cooler and dryer ambient air.
Fans are provided within the greenhouse to generate air movement. As our laundry dries
faster when there is a warm wind blowing, so biosolids will dry faster when there is air
movement in the greenhouse. The drying air must be moved over the wet surfaces of the
biosolids particles.
When we dry our laundry, we spread out the sheets to create a large exposed surface area.
The laundry will dry faster when we turn it frequently from one side to the other. The same
applies to the biosolids in a greenhouse. The sludge to be dried will need to be spread out on
the entire floor as an even layer. The spread out sludge layer on the greenhouse floor must
be turned over again and again in order to constantly present the wet substrate to the
surface and mix the dried particles into the mix.
What is the difference between a solar dryer and a drying bed?
The ancestor of solar dryers is the sludge drying bed. Drying beds were selected because
they are simple and cheap open structures. The drying bed design utilizes an outdoor drain
bed constructed of gravel and grit.
The name drying bed is somewhat misleading because the original design function was for
sludge thickening through drainage. However, depending on the retention period and the
weather, the drying bed could provide some additional thermal drying benefits. The sludge
on the beds would be irradiated and warmed while the sun shone and a warm and dry wind
blew over it. While this had some benefical effect, it took a long time until crevices opened at
the sludge surface and water could evaporate from below.
But when it rained, the biosolids became wet again. And when it poured, there was always
the danger that some sludge could be flushed away. Drying beds can still be useful in
regions with an arid climate.
Sludge drying beds of yonder …
Drying beds can often cause an odor nuisance because the applied liquid sludge may
become anaerobic so that hydrogen sulfide (H2S) is generated and emitted.
Biosolids from wastewater treatment plants contain pathogens. Wet biosolids attract
pathogen vectors, such as flies. Drying beds were thus a perfect breeding ground for flies
and other insects.
… replaced by a solar dryer
Greenhouses provide weather protection. They also serve as an enclosure to prevent odor,
pathogens or dust. If necessary, the exhaust air can be treatedusing an air scrubber.
Because it is a controlled process,solar drying avoids conditions generating odor or dust, and
prevents breeding of flies and other pathogen vectors.
Solar dryers are more effective than drying beds fbecause they provide:
protection from precipitation;
temperatures in greenhouses are far higher;
controlled ventilation of the greenhouse depending on air temperature and humidity;
controlled air flow within the greenhouses for good contact between the drying air and
the surface of the biosolids layer;
supply of mechanically dewatered biosolids limits the required evaporation capacity;
by blending of fed biosolids with dried product a granular, open-porous sludge layer
with a large surface is generated;
frequent turning and moving of the biosolids layer provides for sufficient aeration to
prevent anaerobic conditions and ensure even dryness in the layer.
What functional requirements should a solar dryer meet?
In order for a solar dryer to effectively function and produce the desired results, the system
must be capable of creating function where
the biosolids layer needs to be kept under aerobic conditions to prevent odor
the degree of drying must be controlled to prevent uneven dryness (under- and overdrying); drying beyond 85 % dry solids could cause dust generation;
all particles of the biosolids need to be frequently turned over and moved to prevent
breeding of pathogen vectors;
the operation should be automated to avoid the need for personnel to frequently enter
the greenhouse except for infrequent maintenance.
How HUBER solar dryers accomplish the optimum condition for effective
The SRT Solar Active Dryer is designed in a rectangular footprint. Dewatered biosolids are
fed at one end of the greenhouse. The SRT has the option to have the dried product
removed at the opposite end of the drying bed or, through the use of the turning mechanism,
the dried product can be brought back to the same point of the wet sludge feed for feed and
extraction at one end. This provides the designer flexibility when faced with specific site
The footprint of the greenhouses depends on the required water evaporation capacity,
climatic conditions and whether additional heat is supplied. The greenhouses can be up to
492.1 ft (150 m) long and up to 36.1 ft (11 m) wide. A single greenhouse thus has an
evaporation capacity of up to 492.1 ft *1* 36.1 ft * 951,000 Btu per ft² and per y / 1,547.72
Btu/lb ≈ 10,915,627 lb ≈ 5,458 short tons (150 m * 11 m * 3,000 kWh/(m²·a) / 1.0 kWh/kg ≈
4,950 metric tons) per year where the conditions are favorable, such as in the sunny and arid
South-Western states. For higher capacity several greenhouses are built side by side.
The dewatered biosolids can be either supplied to the SRT Solar Active Dryer in batches with
wheeled front loaders or can be fed by a screw conveyor as the sludge is processed out of
the mechanical dewatering device (such as a Huber Q-Press). If the mechanical dewatering
device is located near enough to the dryer, the conveyor design provides an excellent
solution that does not require an operator to enter the drying zone with a front end loader.
Because the SRT Solar Active Dryer is a linear feed design, dewatered sludge can be added
to the drying floor as it is mechanically dewatered. This is not possible with batch operated
design greenhouses.
Sketch of a solar dryer with additional underfloor heating and heat pump
The above sketch shows a screw press for biosolids dewatering and a screw conveyor for
distribution of the dewatered biosolids cake at the front end of the greenhouse. Key to the
SRT design a travelling bridge turning and mixing device automatically moves the sludge
linearly from one end to the other as it is progressively dried. The turning device moves on
lateral tracks back and forth between both ends of the greenhouse. This design provides for
the frequent turning and moving of the biosolids layer. As a result an operator does not have
to enter the greenhouse to turn or move the solids as it drys., In addition to this function the
turning device can be programed for additional tasks such as back mixing of dried sludge
into the newly added sludge or as a transport mechanism to bring back fully dried sludge to
the head of the greenhouse for removal.
A common sequence used is where the biosolids gradually travel to the far end of the
greenhouse and are dried on their way. From the far end a portion of the dried biosolids is
removed via another screw conveyor and dropped in a dumpster. Another portion is returned
by the travelling scooper back to the front end and blended with supplied dewatered
The sketch also shows an optional underfloor heating system available for the SRT Solar
Active Dryer. In depicted example the treated wastewater effluent heat is recovered at the
plant discharge with a heat pump. The captured secondary hot-water loop is then brought to
a distribution header, distributed immediately beneath the drying floor to provide additional
supplement heat, effectively providing a reduction in footprint required to achieve the same
drying result.
Heat pump recovering heat from the plant’s effluent
Water distributor of an underfloor heating system
Atmospheric control is achieved by automatically controlled variable speed fans mounted at
the ridge of the greenhouse. Louvers are provided at the opposite ridge. The ventilation of
the greenhouse is controlled depending on air temperature and humidity in the greenhouse
and in the environment.
Additional fans are installed under the roof and blow warm air vertically onto the biosolids
layer. About 10.94 ft³/(ft² floor area * min) (200 m³ of air/(m² floor area *h) are circulated..
air circulating
ventilating fans
air admitting
Diagram showing major elements of a HUBER solar dryer
The key element: our travelling bridge turning device
Frequent turning of the biosolids layer is for a key factor for the effective performance of solar
sludge dryers. The surface of the layer must be frequently renewed to provide contact
between the drying air and still moist biosolids. Larger biosolids pieces must be broken into
smaller granular particles with a larger surface. The depth of the layer must be even and
controlled; it should not exceed 1 ft (30 cm) . The biosolids must be gradually moved through
the dryer.
The SRT travelling bridge turning device travels back and forth through the greenhouse. Its
bridge spans the width of the greenhouse and it is guided on tracks along the side of the
greenhouse structure. Its S-shaped scoop rotates and shovels biosolids from its front and
drops them at its back. The design provides for a tight clearance between the turning
device’s scoop edge and the floor so that the biosolids are properly removed from the floor.
As the turning device travels forward, the biosolids are returned a few inches forward from
where they were scooped up, in this way they are gradually moving forward. The velocity of
this forward movement is controlled through the travelling speed of the bridge.
Our travelling bridge turning device
Our turning device has a turning capacity of up to 590 ft³/min (1,000 m³/h) . The entire sludge
layer in the greenhouse can be turned over several times per hour.
The turning device has an additional function: it scoops up loads of biosolids and carries
them along the greenhouse. It can return dry product from the far end back to the front end
and then mixes them with moist freshly fed biosolids. In this way a granular and open-porous
sludge layer is already formed right at the front end. Such a solids structure is important to
prevent formation of anaerobic zones and to avoid odor generation. The sludge surface is
increased and the drying process accelerated.
Our travelling bridge turning device in action
Creates an even biosolids layer for optimal drying
Dryness along a greenhouse
Graphs of biosolids dryness along the length of a greenhouse
blending with returned
Dryness [%]
Position [m]
Graph of biosolids dryness along the length of a greenhouese
The above graph shows a dried solids ratio of about 35 % at the feed end. Such a high
dryness is achieved by blending the dewatered biosolids, which typically have a dried solids
ratio between 20 and 25 %, with returned dry product. Biosolids with 35 % dryness are
granular and form an open-porous structure layer.
As the biosolids are gradually moved through the greenhouse, they are gradually dried
further until they reach the desired dryness. Based on site and climate conditions and design
of the SRT, it is possible to achieve results ranging from 75 % and over 90 % DSR that can
be achieved at the dischargeend of the unit.
What are the benefits of additional heating?
Solar drying depends on sunshine. A few days or even one or two weeks of poor weather
during the summer are not a problem, but drying during winter is slow and unreliable.
The biosolids storage capacity of solar dryers is limited. The thickness of the biosolids layer
is limited to 1 ft (30 cm) . If the sludge layer in spring exceeds this limit, anaerobic conditions
can occur in biosolids that are compressed by the heavy weight of a high layer. Anaerobic
conditions lead to gas generation, odor and safety risks. They must be avoided.
For this reason a considerable portion of liquid sludge or dewatered biosolids needs to be
stored from fall to spring. This requires both a large storage and dryer capacity.
Heat addition reduces the need for storage. When heat is supplied, some drying occurs even
during cold and foggy winter days. Only low temperature heat is required. This permits the
use of waste heat, e.g. from a heat and power cogeneration system at the treatment plant.
Additional heating is not only useful during fall, winter and spring, but even (though to a
lesser degree) during summer (example: rainy summer days).
Where additional heat is supplied, the dryer and the storage facilities can be built smaller.
A simple low cost system uses radiators to heat the circulating air in the greenhouse, but
such a system has a limited energy efficiency. For example, during summer the drying air is
already warm resulting in the driving temperature difference between the heating water and
the air being small. In addition a portion of the supplied heat is lost through the roof and walls
of the greenhouse.
A more efficient solution is underfloor heating: Almost all the supplied heat is transferred into
the biosolids layer; this is exactly where it is needed. The air temperature in the greenhouse
is only slightly raised and the heat losses are small.
Investigations by the University of Braunschweig, comparing dryers with additional underfloor
and radiator heating systems with the same evaporation performance, have shown that the
air temperature in the dryers with radiator heating needed to be about 86°F (30 K) higher
than the air temperature in dryers with underfloor heating.
Radiator heating requires hot water temperatures between 140 and 194°F (60 and 90°C)
while temperatures between (113 to 131°F) (45 °C and 55 °C) are sufficient for underfloor
Installation of an underfloor heating system has a higher capital expense than radiator
heating, but provides the better investment across the design life because it is appreciably
more energy efficient.
When considering in-floor heating it is important to note that the floor should not allowed to
exceed certain temperature thresholds. Limitation to a water temperature of 131°F (55 °C) is
necessary because:
a higher biosolids temperature could lead to odor emission (e.g. due to ammonia
a higher floor temperature could lead to the formation of cracks in the floor.
higher drying air and biosolids temperatures would lead to rising heat losses via
convection and radiation. The dryer’s performance would not rise further
proportionally with the temperature.
Around 63 Btu/(ft²*h) (200 W/m²) of additional heat can be supplied. More heat can be
supplied in exceptional cases, but then the heat losses would increase and it has to be
checked whether an exhaust air treatment has to be applied.
While higher levels of dryness can be expected during certain cycles during the year, even
with additional heat supply, no more than 75 % dryness can be achieved during winter.
Rear end: dried product slides to a lower floor wherefrom it can be removed with a
front loader.
Granular product with a bulk weight of 50 to 55 lb/ft³ (800 to 900 kg/m³)
Biosolids feeding and removal options
Biosolids feeding and removal can be done at opposite ends of a greenhouse (pass through
system) or at the same end (return system). A return system offers the advantages that
operation and maintenance work is needed only at one end and that road access is only
needed at one end only.
The dewatered biosolids can be supplied with a wheel loader or via a screw conveyor.
Where wheel loaders are used, the greenhouse needs to have a certain height so that they
can drop their load onto the biosolids layer. The dried granular product is also removed either
with a wheel loader or via a screw conveyor
Pass through systems
removal with wheel loader
feeding via screw conveyor
removal with wheel loader
feeding with wheel loader
removal via screw conveyor
feeding with wheel loader
removal via screw conveyor
Feeding and removal options for a pass through system
Return systems
feeding with wheel loader
removal via screw conveyor
feeding with wheel loader
removal via screw conveyor
feeding via screw conveyor
removal with wheel loader
Feeding and removal options for a return system
When evaluating solar drying systems, some systems can only be operated in a batch mode.
The SRT Solar Active Dryer is designed as a linear feed technology (quasi-continuous
Batch systems are filled with dewatered biosolids and the dried product is removed when it is
sufficiently dry.
Quasi-continuous systems are periodically fed with dewatered biosolids and some dried
product is also periodically removed. This mode of operation has the advantage of quite
constant operational conditions and avoidance of extreme conditions, e.g. anaerobic
conditions in wet biosolids and thus gas formation, or dust generation when the product
becomes too dry. The biosolids are fed as they are dewatered; no intermediate storage is
needed. This makes operation of the wastewater treatment plant easier.
Screw conveyor for biosolids feeding
Is solar drying odorous?
Little odor is emitted from solar dryers if the fed biosolids are properly stabilized (i.e
aerobically or anaerobically digested) and if anaerobic conditions within the biosolids layer
are avoided. Blending of the fed humid solids with dried product and frequent turning avoids
odor generation. Means for odor control are thus usually not required.
However, strong odor can be emitted if the wastewater, or a substantial portion thereof, is
effluent from certain industries (see below), or if the biosolids are not properly, stabilized.
The following criteria need to be considered when the potential for odor generation is
type and origin of the biosolids; raw sewage sludge is odorous; if wastewater from
certain industries is treated, their potential for odor generation should be considered,
dry solids ratio and volatile solids ratio of the fed biosolids; the higher the dry solids
ratio and the lower the volatile solids ratio, the less odor is generated,
storage of the biosolids before drying; odor can be generated if anaerobic conditions
occur during storage; dewatered biosolids should be immediately fed into the dryer
without further storage; the older the biosolids are, the more odorous they become,
feeding of the biosolids; screw conveyors keep the biosolids open-porous while
pumping would compress them,
the temperature in the biosolids layer should not exceed 120°F (50- °C); accordingly
the heating water temperature for underfloor heating should be limited to 131°F
blending of the fed biosolids with returned dried product; an open-porous layer of
granular biosolids should be generated to prevent anaerobic conditions,
frequency and quality of turning the biosolids layer; it should to be kept open-porous
and aerated.
If one or several of the following criteria indicate that odor control is required, installation of a
cross-flow air scrubber with low flow resistance should be considered. Where scrubbing
should not be sufficient, an additional biofilter could be provided.
Possible scenarios where additional odor control might need consideration:
A considerable portion of the wastewater is effluent from a slaughterhouse or from
leather, paper, food, starch, vegetable oil or soy processing industries; generally high
loads of proteins or fats can cause odor problems,
the fed biosolids are insufficiently stabilized,
the fed biosolids have a pH value below 6.5 or above 7.5,
the fed biosolids are partially hydrolyzed (e.g. during extended storage under
anaerobic conditions).
How are solar dryers sized?
The mass of water which needs to be evaporated in a dryer (ṁw,ev) is calculated with the
following equation from the mass of dewatered biosolids which are fed into the dryer (ṁbs,in),
their average dry solids ratio (DSRbs,in) and the required dry solids ratio of the dried product
(DSRbs,out). It is hereby assumed that the dried solids mass remains the same, i.e. that very
little degradation of organic solids occurs during the drying process. This assumption is
generally justified because biological activity is low in dry solids.
A solar dryer’s evaporation capacity depends on the seasonal irradiation energy entering the
the greenhouse as well as on the temperature and humidity of the drying and ambient air.
The irradiation energy depends on local climatic conditions, the effective glass area through
which irradiation occurs and its transparency for the irradiation spectrum. There are heat
losses through the walls and the roof, depending on the temperature difference between the
drying air in the greenhouse and the ambient air as well as on the thermal insulation of the
greenhouse. Especially during clear nights there are irradiation losses. Additional heat supply
needs to be taken into account.
A certain surface area of the biosolids layer in the greenhouse is needed for the exchange of
enthalpy from the air to the biosolids and evaporated water from the biosolids to the air. The
evaporation rate from a water surface into air is calculated with the Penman formula. The
evaporation rate depends on the temperature and humidity of the air and of the biosolids as
well as on the air velocity. We have modified the Penman formula to match our numerous
empirical results from existing solar dryers.
NOTE: If you, the reader, are involved with the design of a solar dryer we strongly encourage
you to discuss with us how we go about properly sizing a solar dryer. We will work with you
and use your site specific parameters, run this through our computer model, and supply you
with a design report that you can use to support your layout.
The design of solar dryers depends on the following local conditions:
maximum mass of biosolids per year,
type of biosolids (e.g. aerobically or anaerobically digested),
influence by certain industrial effluents (e.g. high loads of protein or fat),
average dry solids ratio of the dewatered biosolids,
required dry solids ratio of the dried product,
climatic conditions (average monthly data of irradiation power, temperature and
additional heat supply (source, capacity, temperature),
storage capacity for liquid sludge or dewatered biosolids,
biosolids feeding and removal regime (see below),
type of greenhouse (e.g. shape and type of glass, see below).
Structural considerations
The following picture shows two of several options for roof shapes.
The cover can be made of glass, polycarbonate sheets, or PE bubble foils. Key
characteristics are their transparency for sunlight and thermal insulation against convective
heat losses. Important data are weight per unit area, cost per unit area and durability. Their
Aesthetic appearance needs to be taken into consideration. The choice of the material also
depends on local conditions, such as wind or snow loads. Resilience against heavy rain- or
hailstorms must also be considered.
Two options for roof shapes
Roofing materials for greenhouses and their characteristics
twin wall sheet
PC4, highly-UVstabilized
Weight per
unit area
Guaranteed life
0.27 (1.3)
0.78 – 0.82
0.63-0.67 (3.6
– 3.8)
2.04 (10)
0.88 – 0.91
1.13-1.16 (6.4
– 6.6)
0.084 (0.41)
0.81 – 0.83
0.51-0.63 (2.9
6 mm (1/4“)
Single sheet
safety glass
4 mm (10/64“)
PE bubble foil
8,5 mm (22/64“)
A considerable amount of energy radiates into the outer space during clear nights. It is well
known that nights in a tropical desert can be very cold. For this reason it should be
considered whether the greenhouse could be provided with automatically shutting blinds.
Can solar dryers produce Class A biosolids?
Class A biosolids are disinfected and used for land application without restrictions. They are
e.g. used on golf courses, in parks or gardens. Especially valuable are dried Class A
biosolids of exceptional quality which can be sold in bags.
Requirements for Class A Biosolids are stated in EPA Rule 503 and can be found in “A Plain
English Guide to the EPA Part 503 Biosolids Rule” (September 2003). The following
conditions must be met:
Precondition for Class A biosolids is that specified heavy metal concentrations are not
Class A biosolids must have been subject to a process for further pathogen reduction.
Alternative A specifies requirements for thermal biosolids treatment. Regime A
applies for solar drying:
Biosolids with 7 % solids or greater must maintain a temperature of minimum 122°F
(50°C) for at least 20 minutes. The following time-temperature relationship must be
D ≥ 131,700,000 / 10(0.14·T)
whereby D = duration in days and T = temperature in °C.
Examples: if T = 50 °C → D = 13 days; if T = 70 °C → D = 0.5 hours
The temperature and its duration must be monitored and recorded for proof that the
required conditions have been met.
Either fecal coliforms or salmonella are used as indicator organisms to monitor the
product’s disinfection. The concentration of fecal coliform colonies must be below a
most probable number (MPN) of 1,000 per gram dried solids or the concentration of
salmonella in the biosolids must be below a MPN of 3 in 4 grams of dried solids. The
relevant authority prescribes where samples shall be taken and how frequently.
Another requirement is that the dried product must have been subject to a process
reducing vector attraction. Such vectors are flies, mosquitoes or birds. This
requirement also serves the purpose to prevent pathogens from re-growing. If the
biosolids have been properly stabilized prior to their drying, the requirement is met.
Such stabilization processes are usually aerobic or anaerobic digestion.
The requirement for proper anaerobic digestion is degradation of minimum 38 % of
the volatile solids in the raw sludge or a further degradation in the digested sludge of
no more than 18 % in lab tests at minimum 30 °C during 40 days.
The requirement for proper aerobic stabilization is that its oxygen uptake rate at 20 °C
does not exceed 1.5 mg per hour and gram of total solids.
These requirements are usually easily met by sludge stabilization systems.
Contrary to common understanding,there is no additional requirement for the
product’s dryness. A product temperature of 176°F (80°C) and a dry substance
content of 90 % is only required where non-stabilized solids, e.g. raw sludge, are fed
into the dryer (this is Alternative 5 for the treatment of biosolids in a process for
further pathogen reduction according to the prior 40 CFR Part 257 regulation).
How can these requirements be met with a solar dryer?
During summer the drying biosolids in the layer can achieve a temperature of 122°F (50°C)
over a period of 13 days, but this is usually not possible during fall, winter and spring, even if
the greenhouse is provided with additional heating.
The following options are available:
The entire mass of biosolids which have been dried during a year is stored for
another year in a non-ventilated additional greenhouse, wherein they are baked
during the following summer.
The dried product is disinfected in a subsequent heated reactor. The temperature of
the biosolids is maintained for a minimum of 20 minutes at minimum 122°F (50°C),
but the time-temperature relationship still needs to be meet.
Does the dried product meet Class B requirements?
A product is Class B if the geometric mean of the fecal coliform density of 7 samples is less
than 2 million MPN per gram of total solids.
No such testing is required if the biosolids have been properly anaerobically stabilized in a
digester at a minimum temperature of 35 °C and with a mean cell residence time of 15 days,
or if they have been aerobically digested with a mean cell residence time of 40 days at 20 °C
or 60 days at 15 °C.
If the biosolids did not yet meet Class B requirements before their drying, they certainly will
after additional drying. This can be demonstrated via fecal coliform analysis.
Is solar drying labor intensive?
Feeding dewatered biosolids and removing dried product with a wheel loader requires
about 30 minutes per greenhouse and day. Where the biosolids are fed with screw
conveyors, only 5 minutes per unit and day are needed. During summer biosolids are
fed every day and during winter usually only once per week.
The appearance and height of the biosolids layer should be monitored every day.
This work is done in about 10 minutes per greenhouse.
A more thorough inspection once per week requires about half an hour per unit.
Settled dust within the greenhouse must be periodically removed. The frequency of
such work depends on the amount of dust generated. 2 hours per unit and month are
usually sufficient to do this work.
Monthly maintenance of the equipment is limited to inspection and lubrication of
motors and bearings. The throughput and quality of the product is monitored. This
takes 2 to 3 hours per unit and month.
Once per year the greenhouse and its entire equipment is thoroughly inspected and
cleaned. This requires about 12 hours per unit and year.
Our solar dryer reference installations in the US
We supplied a solar dryer to Tooele, Utah. The customer is very happy with its performance.
Class A biosolids are generated during summer. Another Huber solar dryer is presently being
constructed in California.
Huber solar dryer in Tooele, Utah
Solar biosolids drying is an efficient, effective, economical, and reliable process.
Solar energy is free to harvest where sufficient space is available.
The dried product can be easily stored and then used as valuable fertilizer for land
application (where required as Class A product). Where this option is not available, the dried
biosolids can be used as fuel for incinerators.
We provide dependable information for the design of solar driers, based on specific site
Conversion factors:
1 m³/(m²*h) = 0.0547 ft³/(ft²*min)
1 m³/h = 0,588578 ft³/min
1 kWh = 3,412.14 Btu
1 W/(m²*K) = 0.17611 Btu/(ft²*°F*h)
1 kWh/m² = 317 Btu/ft²
1 kWh/kg = 1,547.72 Btu/lb
1,000 kWh = 3.4121416331 MM Btu
1 short ton = 0.907 metric ton = 2,000 lb
7,000 grains = 1 lb
1 ft = 0.3048 m
1 ft² = 0.0929 m²
1 ft² = 2.29568*10^-5 acres
1 m² = 0.000247105 acre
1 ft³ = 0.0283 m³
1 kg/m³ = 0.0624 lb/ft³
1 kg/m² = 0.204816 lb/ft²
HUBER Solar Active Dryer SRT
– True backmixing of sludge for a perfect drying bed
without odour or dust generation
– Maximum flexibility of sludge feeding and removal,
optionally even on the same end
– Modular system providing for the option of fully
automated operation
– Optimally suitable to be combined with floor heating
➤ Solar biosolids drying
There are many good reasons for biosolids drying:
➤ Reduce disposal costs due to mass reduction
➤ Produce storable and easy-to-handle dried
➤ Open up new disposal options
Our solar dryers combine green technology with easy and
safe operation.
➤ HUBER SRT system
The sludge turner on its way through the greenhouse
The basic principle of the HUBER SRT system is drying of
sewage sludge in a greenhouse using incident solar
radiation and artificially generated wind to evaporate
water from sludge. A special sludge turning system
performs spreading, granulation, turning, mixing and
backmixing of sludge as well as its transport from one end
to the other.
This solution allows for continuous system operation so
that the sludge bed in the greenhouse remains constant.
Due to the special features of the sludge turning
assembly, particularly its backmixing function, an openporous and only slightly wet sludge bed is maintained,
generating neither odor problems nor dust. The sludge is
dry enough to prevent odor-generating biological
processes, but still wet enough to prevent generation of
dust under mechanical stress.
Sludge feeding can be adjusted to suit customer-specific
requirements. Dewatered sludge can be fed into the
greenhouse either manually, i.e. with a wheel loader, or
automatically through special conveyors from the
dewatering system. The dried sludge can be stored at the
end of the greenhouse or forwarded via conveyors to a
loading station.
Free-flowing dried granulate
The produced granulate is easy to handle due to its high
solids concentration. The pea-sized granules are freeflowing.
Automated sludge feeding with a screw conveyor
➤ Sludge turning device
The sludge turner is the core of the drying system and
consists of a rotating double shovel mounted on a
travelling frame. The double shovel fulfils two functions:
➤ Sludge turning: As the sludge turner travels forwards
with the rotating double shovel, the sludge is mixed,
broken up, aerated and transported. The sludge is
completely restacked as the sludge turner travels
from one end to the other. Each individual sludge
grain inside the greenhouse is moved within a short
period of time. This is ideal for a good drying result
and prevents odors.
➤ Sludge transport: The sludge turner takes up some
sludge at a defined point and transports it inside its
shovel to another point. This permits backmixing of
dry sludge into wet sludge. Sludge feeding and
removal can take place at opposite ends or the same
end, as requested.
The rotating shovel of the sludge turner takes up sludge
and spreads it on the sludge bed;
As the turning device gradually travels forward, the entire
bed is mixed and restacked
Maximum flexibility of sludge feeding and removal gives
freedom of design. It is for example possible to build the
greenhouse up to the boundaries of the WWTP grounds
and save space for roads or turning curves.
The sludge turner is made of corrosion resistant stainless
steel and travels on low driveway walls to avoid shadows.
The machine pulls itself through the greenhouse along
chains and is safely guided. The electrical control system
measures and records all relevant parameters. If
requested, these data can be transferred to the main
control station or made available for remote access via
the internet.
Controlled sludge transport from one place to another as
the turner moves with a filled shovel
General view of the system: sludge and air flows
➤ Climate control
Climate probes, ventilators and ventilation flaps are
installed in the drying plant to ensure ventilation at the
right time and to generate sufficient air flow on the sludge
surface. Ventilation is regulated on the basis of continuously measured water absorption capacity of outside and
inside air; excessive water condensation is prevented.
Ventilators blow dry air over the bed of freshly turned
sludge. The climate control system uses not only
theoretical calculations, but also empirical operation and
measurement data.
➤ Seasonal climate and
external heat sources
Backmixing and aeration of sludge
Drying efficiency depends directly on the climatic conditions with less water being evaporated in winter than
in summer. Different strategies can be applied to process
continuously generated sludge volumes:
➤ Operator accepts greatly varying product dryness and
selects sludge disposal options accordingly.
➤ A sludge buffer tank is used for sludge storage in
winter and emptied in summer when dryer performance is high.
➤ Solar drying is supported with external energy sources
in winter.
➤ Several of these options are combined.
An eco-friendly method of supplying additional energy is
the use of a heat pump, which lifts thermal energy,
extracted through heat exchangers from the WWTP
effluent, to a higher temperature so that it can be used
for sludge drying. Other heat sources can also be used as
available (e.g. exhaust heat).
Ventilators blow dry air over the sludge bed.
Supply of additional heat through highly efficient floor
heating ensures maximum heat transfer with minor
losses. Efficient evaporation permits space saving system
Greenhouse with ridge flap for climate control
9735 NorthCross Center Court STE A · Huntersville, NC 28078
Phone: (704) 949 - 1010 · Fax: (704) 949 - 1020
[email protected] ·
Subject to technical modification
0,15 / 1 – 9.2010 – 10.2007
HUBER Solar Active Dryer SRT
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