Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Improvement of the evaporation phenomenon as
a waste management technology
E.A. Duarte*, I. Neto*, J. Branch
"S.A.Q.A., Institute Superior de Agronomia, Tapada da Ajuda,
1399 Lisboa Codex, Portugal
^Departamento de Matemdtica, Secgao de Estatistica, Institute
Superior Tecnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
Abstract
The problems concerned with waste management abound in most regions of the
world, and so, efforts must be made to assess the global level of pollution and to
understand the major factors involved in order to propose efficient waste
treatment technologies. This must be compatible with environmental and
economic conditions and appropriate to semi-urban settlements, isolated
communities and a variety of industrial situations. The main purpose of this
paper is to present a recent waste management technology as an alternative
solution to traditional treatment systems in regions where those systems are not
economical or technical feasible, using for this purpose the advantages of
climatic conditions (air temperature, relative humidity and wind speed).This
integrated technology combines two different processes: i) optimisation of the
solid/liquid separation, using a decanter; ii) elimination of the liquid fraction of
the effluent by the utilisation of evaporation panels, avoiding the ground water
contamination. In this way the evaporation phenomenon, witch is so important
and resolute in certain cases, is optimised and should be exploit intensely. The
authors carried out a research program that includes a set of experimental trials
on an evaporation plant implemented for treat wastes from a pig unit, located in
a high polluted area of Portugal. In that study, both the volume of evaporated
liquid and the relevant environmental variables were simultaneously measured
(air temperature, relative humidity, wind speed) to be used in a mathematical
model immediately applicable both for optimising plant management and project
engineering of plants, performance parameters. The conclusions of this research
program emphasised the importance of the evaporation phenomenon as a simple
solution to deal with complex environmental problems.
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
340
Water Pollution
Introduction
The treatment of waste generated in agro-industrial activities has been a
perplexing problem for a long time.
The aim of this paper is to present the study of the improvement of the
evaporation phenomenon as an alternative solution to the treatment of high
polluted wastes, mainly in regions where is difficult to implement traditional
technologies.
Our study presents the results of a three years research program, developed
in a "pig manure unit without land", where the evaporation technology
combined with an efficient solid/liquid separation was evaluated.
This technology allow to avoid the contamination of natural receptors (soil
rivers, sea) by uncontrolled discharges of this high polluted wastes, using for
this purpose the relevant environmental variables (air temperature, wind speed
relative humidity).
This fact is very important in countries where the climatic conditions are
favourable and, at the same time the noble energies have a high cost.
Material and Methods
Lay-out of the treatment plant developed
The treatment plant implemented was dimensioned to treat lOrn^ of influent
daily.
Figure 1 shows a scheme which demonstrate the final lay - out of the
treatment plant developed.
FIC.l - LAY-OUT OFTUKTKKATJMKNT PLANT DICVKLOPKI)
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
341
The influent, coming from the pig manure unit, is collected in a
Homogenisation Tank (A), with a capacity of 30 m^; in this tank was installed a
mixer to keep the influent perfectly homogenised.
A pump conduct the influent to a separation system - Decanter (B) - where
an efficient solid/liquid separation is performed.
The solid fraction of the influent is collected and storaged to future
stabilisation.
The liquid fraction obtained is conducted and storaged in a Storage Basin
(C), with a 500rrr of capacity; this basin was previously impermeabilized to
avoid soil contamination.
The reason for the storage of liquid fractions is connected with the treatment
selected - evaporation panels - which is related with the fact that the climatic
factors are changeable all over the year.
The liquid fraction feeds four Evaporation Panels (D), placed to take
advantage of the prevailing wind, to order to optimised the evaporation of the
water contained in the liquid fraction of the influent.
This feed process is achieved by a pump system that load a small tank Evaporation Plant Feed tank (E) - where another pump allows the irrigation of
the panels by an aspersion system.
The concentrated fluid, that result from the evaporation phenomenon, is
conducted to the homogenisation tank, to be incorporated with the fresh
influent, for following solid/liquid separation.
All the treatment plant is controlled by an automatic system.
The relevant environmental variables - air temperature, relative humidity and
wind speed - on the evaporation phenomenon are measured and registed in a
meteorological digital central, connected with a memory-card system that
storage the data to future analysis.
The reason for the choice of evaporation panels
The phenomenon of evaporation is so important and resolutive in certain cases,
that it is absurd not to exploit it intensely.
The principles of this technology is based on the mass transfer between a nonsaturated air flow and the waste water film establishing inside of an evaporation
panel used to concentrate the agro-industrial waste waters.
The idea of creating large lakes is unproposable, as land is too precious for
us.
The surface water of the large lake can be arranged on various floors
separated from one another so that the air required for evaporation can pass
through, and therefore we would obtain an equal surface area exposed to the air,
with a smaller surface occupied, proportionately to the layers which will be
created.
The evaporation panel, with a double pit structure, can develop approximately
600 m2 of surface area, in which the real area occupied is equal to 3.5 m^ (2 x
3.3 x sin 30 degrees = the width of the panel x the projection of its height of 3.5
m on the land).
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
342
Water Pollution
But considering an area of 4 m^ due to the bearing structure and the panel
thickness, which correspond to 0.67%, 1/150 of the previous surface.
The result may be considered decidedly satisfactory, mainly if we consider
that we have succeeded in obtaining these reductions, still leaving "channels" in
which the air can pass, which only produce a loss in the load of 4mm of c.d.a.
Each panel is 2 x 3.5 x 0.4 m (breadth x height x thickness) in size. It is built
by putting together 4 blocks, size 1x3.5x0.2m each.
These blocks are held together by a metal structure made of galvanised steel,
so as to form a single, compact and rigid unit.
The panel is placed on the ground on its breadth, and it is tilted at a 60 degree
angle with the bearing surface.
In the middle of the panel, at approximately 2/3 of the height from the base, a
galvanised steel pipe passes through the panel.
This pipe will carry the liquid fraction in to the panel. On the top of the pipe
there is a particular type of outlet, by means of which it is possible to wet the
entire panel
By means of a particular pump, at present working and resting times, the
panel will be sprayed with the liquid to be evaporated.
The position of the panel must be such as to offer maximun rear-surface to
the direction of the prevailing wind.
Once the liquid fraction has been sprayed, it forms a veil of water on the
entire surface of the panel, any excess water falls back into the overflow well or
into the aspiration tank, while the veil of water is over by the air (which at that
moment will have a given temperature, humidity and speed) which will absorb
the amount of water required to reach saturation (humidity = 100%), and in this
manner EVAPORATION begins.
Methodology
During this study was been evaluated the reability of the equipment and all the
operational problems, especially those of the instrumentation, control and
automation, and also repair and maintenance periods.
Full scale design evaluation starts with data collection and their evaluation.
There must be enough data for the influent and effluent quality of biological
stages for the whole treatment plant.
Due to many possible mistakes the data collected often are incorrects even if
chemical analysis is reliable.
One of the most promising ways to prove the data is to make mass balances,
especially for total solids (TS), volatile solids (VS), total suspended solids
(TSS), chemical oxygen demand (COQ) and biological oxygen demand (BOD).
After having corrected the data, the next step is to calculate the real treatment
efficiency. For this task it is necessary to analyse the specific situation during the
evaluation period, especially in regard to operational parameters, control
strategy, unusual conditions and their impact.
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
343
Results
The tests were performed over the period from February to October 1994.
Table I shows the average values of the control and performance parameters
obtained during the study period.
TABLE
I - Study period average values of the control and
performance parameters from Feb. to Oct. 1994
PARAMETERS
HT
(INFLUENT)
SB
(LIQ.
FRACTION)
SEPARATION
REMOVAL
pH
7,3
7,7
-
TS
VS
TSS
COD
BOD
CV
N-NK
N-NH/
N-NOj
CT/N
PiOj
Na
K
Ca
Mg
Fe
Mn
Cu
Zii
8,49 g L '
5,06 g I/
4,63 gi;'
9900 mg L"'
5200 mg I/
2,49 g L"'
1197mgl/
993 mg I/'
19,3mgL"'
2,47
329 mg I/
241 mgl/
1177 mgL-'
320 mg L^
144 mg L'^
60 mg I/*
5,6 mg I/
8 mg L"'
lOmgl/
5,45 g L'^
3,71 gL"'
l,43gl/
4098 mg I/'
2031 mgL"'
2,15 gL"'
820 mg L"'
668,7 mg L"'
8,37 mg I/
1,89
134 mg L-'
240 mg I/
890 mg L"'
218 mgL''
88 mg L"'
18 mgL'*
2,0 mg L"'
4,0 mg I/*
6,0 mg L"'
39%
26.7%
69,1%
58,6%
60,9%
_
31,5%
32,7%
56,6%
_
59,3%
0,41%
24/4%
31,8%
38,9%
70%
64,3%
50%
40%
SOLID
FRACTION
(HzO) 6,8
(KC1) 6,3
22,3%
15,7%
_
_
_
9,1%
0,57%
0,36%
_
16
1,47%
0,24%
1,42%
0,17%
0,032%
0,008%
0,046%
0,002%
0,003%
SOLID
FRACTION
RECOVERY
40%
27.3%
_
_
_
27,3%
369%
_
_
_
605%
598%
39,9%
_
_
-
Samples of influent, liquid fraction and solid fraction were collected twice a
week during the study period and its main characteristics were analysed by
analytical methods refereed in AO AC [1] and Standard Methods [4]. This
main characteristics were the pH, the concentration on total solids (TS), volatile
solids (VS), total suspended solids (TSS), chemical oxygen demand (COD),
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
344 Water Pollution
biological oxygen demand (BOD), total carbon (Cj), forms of nitrogen,
phosphorous and mineral elements.
Table I also shows the separation removal and solid fraction recovery obtained
on thefirststage of the treatment - Separation process.
Table II shows the evaporation performance according with the relevant
environmental variables of the treatment - Evaporation Panels.
TABLE II - Evaporation performance according
environmental variables of the technology implemented
Environmental variables
Month
T
RH Wind
(%) Speed
CO
(m/s)
143 75.1
2.7
Feb
2.4
126 820
Mar
15.0 65.0
2.8
Abr
17.5
720
2.7
May
Jim
2.7
22.5
61.0
50.1
3.4
25.2
Jul
61.0
23.5
3.0
Aug
70.0
21.5
3.0
Sept
17.5
73.2
3.1
Oct
to the relevant
Evaporation performance
nvVmonth/
Average
m^/day/
(nrVmonth/
panel
panel
panel)
1.36
40.72
0.83
25.73
210
63.00
1.67
51.77
8310
70.75
2.77
4.75
147.40
9799
316
230
6915
1.86
5789
The evaporation performance was studied based on a mathematical model that
relates the evaporation rate and the thermo-fluid-dynamic air properties
influencing the phenomenon. The accuracy of the model and the simplicity of
acquiring the dependent variables make the mathematical model immediately
applicable both for optimising plant management, e.g. Bianchi and Catalano [2].
The data collected from the memory card were applied to the mathematical
model to obtain the values of evaporation performance.
Discussion
Three factors were identified which could affect the efficiency of the technology
implemented: influent composition, solid/liquid separation efficiency and
meteorological conditions.
Concerning the composition of the influent we can conclude that is a very
diluted influent (mean values of 8.49 g/L of TS, 5.06 g/L of VS, 4.63 of TSS,
9900 mg/L of COD and 5200 mg/L of BOD) comparing with the values
mentioned in the literature, e.g. Duarte [3].
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
Water Pollution
345
Concerning the separation removal for this diluted influent we achieved very
high performance rates with this separation process, mainly to removal values of
the following parameters: TSS, COQ, BOD, N-NOg" and ?2O5 This good
performance in the first stage of the process allows the optimisation of the
second stage, mainly concerning the efficiency of the evaporation process, which
can be compromised if occurs the colmatation of the pits inside the panels.
Another problem could be the odour emission which is avoid once the
concentration of the liquid fraction, due to the parameters that have the capacity
of produce odours, is very low.
This separation process also achieved a very high solid fraction recovery
which have simultaneously two advantages:
- production of a subproduct with a high agronomic value (N,P,K and C).
- production of a easily storage and management subproduct.
The evaporation performance obtained allowed to evaporate the water
contained in the liquid fraction during the study period, which was a positive
result, once the reserve capacities of the storage basin (SB) were never
overtaken, even with the adverse climatic conditions that occurred during this
period.
Conclusions
The aim of this study was to evaluate the influence of return flows on the
operation of a waste water treatment plant.
This approach considers the treatment plant as a global system, in which
interactions between stages of treatment are very important. It has shown that
every process (solid/liquid separation and evaporation process) can be disturbed
by the return flows and that its impact is such that can flexible the processing
system in order to absorb overloads on the waste water treatment plant.
The main goal of this experimental study was to save construction costs and
to further reduce the space required for the treatment plant by the
implementation of "Evaporation Panels" which allow the elimination of the
water contained in the liquid fraction of the effluent.
Evaporation is surely one of the most interesting technologies for agroindustrial waste waters treating and disposal, specially natural evaporation which
takes place as a result of the air temperature, its humidity and the wind.
Summarising the particular characteristics of the evaporation panels we may
state the following:
a) The evaporation panels are energy saving. The energy utilised with the
same type of waste water treated, is about 1/10 of the amount utilised in the
classic plants.
b) They offer the possibility of utilising renewable energies (air temperature,
humidity and wind) and not the "noble" types of energies.
c) They do not create any problems for disposal in public waste water
disposal plants. With evaporation, the wastes are eliminated in full respect of the
Transactions on Ecology and the Environment vol 7, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541
346 Water Pollution
environment. COD, ammoniac nitrogen, (NO2", NO]-, P) eutrophic
substances no longer exist.
d) Daily and weekly plant operation checks are inexistant.
- all the chemical and physical parameters are completely inexistent - pH, Rh,
O2, mud control with the ImhofF cone, SVI, SSUMA, mud age, bacterial
development etc.
- chemical analysis during the operating phases, and on the discharged water are
inexistent.
- problems related to plant down-time due to the presence of toxic agents,
antibiotics or closing of the plant due to holidays, are inexistent.
- in case of a rapid increase in the polluting load or an increase in production
with a consequent increase in the amount of liquid wastes, the plant is adapted
to the new requirements practically immediately, without any problems
concerning space, new tanks, new complicated plants, etc.
e) Extremely simple technology in the plant, in the electric and hydraulic
parts.
f) It is extremely simple to manage the electro-mechanical parts, compared to
any other type of purification plant.
All these advantages, along with the results obtained naturally with the
evaporation panels, make this technology the NEWEST, the rediscovery of an
extremely natural process, such as EVAPORATION, which from an
"environmental point of view" is a natural treatment carried out with
TOTALLY RENEWABLE ENERGIES.
Acknowledgements
We should like to tank the European Union for sponsored the LIFE Project that
allowed the implementation of the treatment plant and all the studies that we
carried out, since January 1993 to October 1994.
References
1. A.O.A.C. (1980) - Q07cW M?f/%x& q/^Wyj/j of f/?c ^Moc/a//w; of
Official Analytical Chemists. 13th, Ed. A.O.A.C., Washington, DC
2. Bianchi, B , Catalano, P. (1983) - Studio teorico dello scambio di massa in
panelli evaporativi, pp 7-11, VI. Proceedings of the V Convegno Nazionale
A.I.G.B. Maratea.
3. Duarte, E A (1991) - Digestao Anaerdbia e Valorizaqao de efluentes de
siumculiura. PhD thesis. Lisbon, Portugal.
4. Standard Methods. (1980) - For the examination of water and wastewater.
15th., Ed. MacMillan Press.