Filtro Biológico Percolador com as paredes laterais abertas
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3. different configurations for the filter media, packing height
and the investigation of the real need of secondary settlers
(Porto et al. ; Santos ; Silva & Gonçalves ;
Almeida et al. ; Chernicharo & Almeida ; Rodri-
guez et al. ). However, conventional TFs have large
amounts of concrete, which increases their construction
costs. Besides this, nitrification is not always achieved,
with oxygen limitations being one of the reasons for the
poor performance in terms of ammonia removal.
In conventional TF, air circulates through the open sur-
face area at the top of the filter and the side wall openings
that are part of the underdrain system. Good aeration is
essential to maintain aerobic conditions inside the filter
media, promote better removal efficiencies and prevent
bad odours (Metcalf & Eddy ). In many aerobic sys-
tems, the oxygen transfer rate to the cells is the limiting
factor, determining the rate of biological conversion pro-
cesses (Gonçalves et al. ). Thus, efficiency in the
supply of dissolved oxygen (DO) for the bacteria becomes
a dominant factor, especially for the slow-growing nitrify-
ing bacteria. Increasing oxygen transfer is likely to
enhance the filter performance, especially in terms of nitri-
fication (Wik ; USEPA ; Gujer ). In this sense,
removing the side walls is probably a good way of increas-
ing the circulation of air inside the media, thus allowing a
better aeration.
In order to address the needs for improving ventilation
(enhance performance) and reducing the amount of con-
crete (reduce costs), whilst still keeping the inherent
simplicity required for TFs, a new version of TF, without
side walls and false bottom, was conceived by von Sperling
at the Federal University of Minas Gerais, Brazil. Side walls
have been substituted by a simple wired fence supported by
simple pillars made of polyvinyl chloride pipes filled with
concrete. The bottom slab has been replaced by an under-
drain layer composed of coarse stones. Additionally, as a
means of introducing further simplifications, the conven-
tional rotating distributors have been replaced by fixed
distributors (channels with V-notch weirs). Finally, the last
stage of economic savings was the fact that no secondary
sedimentation tanks were included. Because of the absence
of side walls, the unit was called open trickling filter (OTF).
Vieira & von Sperling () present a comparison of con-
struction costs between OTFs units and conventional TF
units, indicating substantial savings for the former (74%).
The objective of this paper is to present a performance
evaluation of the OTF as post-treatment of the sanitary efflu-
ent from a UASB (upflow anaerobic sludge blanket) reactor,
operating with two distinct loading rates.
METHODS
Experimental site and UASB reactors
The research was conducted at the Centre for Research and
Training on Sanitation UFMG/COPASA. The system under
evaluation (UASB reactor followed by OTF without second-
ary settlement stage) receives sanitary sewage from the city
of Belo Horizonte, Brazil, after preliminary treatment
(coarse and medium screens and grit chambers). The
UASB reactors operated with average hydraulic retention
time of 6 h and upflow velocities of approximately 1 m hÀ1
.
Characteristics of the open trickling filter
In order to improve the conditions for nitrification in the
OTF, a packing height of 3.5 m was adopted, which is
higher than the usual height of 1.8–2.4 m for conventional
TFs (Metcalf & Eddy ). This total height comprised
three layers. The top layer (0.5 m) was composed of coarse
stones, aiming at facilitating the distribution of the influent
along the surface area. The middle layer (2.2 m) comprised
the actual filter media, with crushed stones between 38
and 76 mm diameter. The bottom layer (0.8 m), also made
of coarse stones, acted as the filter underdrain. Additional
construction details and cost estimates are presented else-
where (Vieira & von Sperling ). Figures 1 and 2 show
schematics and views of the OTF.
Operation and monitoring
The OTF was operated under two different loading rates.
For the first phase (P1), from 29/09/2010 to 13/07/2011
(nine and a half months), the OTF was operated with an
average flow of 33 m³ dÀ1
, resulting in an average surface
hydraulic loading rate (HLRS) of 4.1 m³ mÀ
² d and a volu-
metric hydraulic loading rate (HLRV) of 1.1 m³ mÀ3
dÀ1
.
For the second phase of operation (P2), from 26/08/2011
to 21/06/2012 (nine months), the OTF was operated with
a mean HLRS of 9.3 m³ mÀ
² d, HLRV of 2.6 m³ mÀ3
dÀ1
and flow of 70 m³ dÀ1
. Both phases (P1 and P2) covered
warm (rainy) and cold (dry) periods.
Monitoring of raw sewage (RS) and of the effluents from
the UASB reactor and OTF was undertaken with simple and
composite samples (24 h – under refrigeration), three times a
week. The physical and chemical parameters analysed were:
pH, liquid temperature (T ), DO, alkalinity (expressed in
terms of CaCO3), total suspended solids (TSS), chemical
2747 P. C. Vieira et al. | Performance evaluation of a novel open tricking filter Water Science & Technology | 67.12 | 2013
4. oxygen demand (COD), BOD, total Kjeldahl nitrogen
(TKN), ammonia nitrogen (N-ammonia), nitrite (NO2
À
-N)
and nitrate (NO3
À
-N). These parameters were determined
according to APHA (), with the exception of NO3
À
-N,
which was determined by the salicylate method described
in Rodier (). The detection limits for nitrate and
nitrite, according to the methods used, were 0.1 and
0.005 mg LÀ1
, respectively.
Data analysis
Removal rates in the two phases of operation have been cal-
culated using the ratio of the removed load in OTF and (i)
the surface area of the OTF for N-ammonia (g N-ammonia
mÀ2
dÀ1
) and by (ii) the useful volume of the OTF for BOD
(kg BOD mÀ3
dÀ1
). Also, for the calculation of the ammonia
removal rate, the specific surface area of the support
medium was used (crushed stones between 38 and 76 mm,
with 60 m² mÀ3
).
Comparison between operation phases (P1 and P2) was
undertaken using non-parametric tests – Mann–Whitney
(Conover ) and Wald–Wolfowitz (Conover ;
Naghettini & Pinto ), with a 5% significance level.
RESULTS
The mean values of the operating conditions during the
evaluation period are presented in Table 1. The mean flow
of 33 m³ dÀ1
, applied in the first phase of operation (P1),
Figure 1 | Cross-section of the open trickling filter.
Figure 2 | View of the UASB þ OTF system for the two phases of operation. p.e: population equivalent.
2748 P. C. Vieira et al. | Performance evaluation of a novel open tricking filter Water Science & Technology | 67.12 | 2013
5. corresponds to an equivalent population of approximately
250 inhabitants, which leads to the following per capita
area and volume requirements: 0.04 m2
of total surface
area per inhabitant and 0.140 m³ of total volume per inhabi-
tant. In the second phase of operation (P2), the mean flow of
70 m³ dÀ1
corresponds to an equivalent population of
approximately 550 inhabitants, which leads to the per
capita area of 0.02 m2
of total surface area per inhabitant,
and volume requirements of 0.07 m³ of total volume per
inhabitant.
According to Metcalf & Eddy (), the loading rates
can be classified as ‘low or standard rate’ for the organic load-
ing rate and ‘intermediate rate’ for the hydraulic surface
loading. Under these conditions, between full and partial
nitrification is expected (Metcalf & Eddy ). However, it
should be remembered that the volumetric organic loading
rates reported by Metcalf & Eddy () are for effluents
from primary sedimentation tanks (usual BOD removal effi-
ciencies around 30%), whereas in the present case the
effluent from a UASB reactor (BOD removal efficiency
around 70%) is used. The loading rates are within those
expected to lead to the joint occurrence of BOD removal
and nitrification in a single stage, reported in USEPA ().
The BOD/TKN ratio of the influent to the OTF, which is
important for nitrification, resulted in values below the maxi-
mum recommended for single-stage nitrifying TF.
Table 2 presents a summary of the effluent concen-
trations and removal efficiencies obtained in each stage of
the treatment line and in the overall system, for each
phase of operation.
Table 1 | Mean values of the loading rates applied to the OTF for each phase of operation
Phase of operation Flow (m³ dÀ1
) HLRS (m³ mÀ2
dÀ1
) HLRV (m³ mÀ3
dÀ1
) OLRV (kg BOD mÀ3
dÀ1
) Influent COD/BOD Influent BOD/TKN
Phase 1 33 (6) 4.1 (0.7) 1.1 (0.2) 0.08 (0.04) 2.7 (0.5) 2.4 (1.1)
Phase 2 70 (8) 9.3 (0.7) 2.6 (0.5) 0.12 (0.04) 2.8 (0.8) 1.2 (0.3)
HLRS ¼ surface hydraulic loading rate; HLRV ¼ volumetric hydraulic loading rate; OLRV ¼ volumetric organic loading rate; ( ) standard deviation.
Table 2 | Synthesis of concentrations and removal efficiencies obtained in the UASB–OTF system for each operational phase
Raw sewage
Effluent UASB Effluent OTF
Parameter/ Concentration (mg LÀ1
) Concentration (mg LÀ1
) Removal (%) Concentration (mg LÀ1
) Removal (%) Global removal (%)
operation phase n Mean Mean Median Mean Median Median
DO P1 51 0.3 (0.4) 0.6 (0.6) – 5.4 (0.6) – –
P2 91 0.6 (0.5) 0.8 (0.5) – 5.4 (0.9 – –
pH P1 70 7.3 (0.2) 6.9 (0.2) – 7.6 (0.2) – –
P2 88 7.3 (0.2) 7.0 (0.2) – 7.8 (0.2) – –
Alkalinity P1 68 196 (42) 214 (34) – 130 (40) 42 42
P2 90 218 (33) 244 (32) – 201 (32) 10 10
TSS P1 56 233 (103) 71 (58) 78 49 (24) 24 83
P2 89 219 (93) 69 (40) 73 66 (47) 14 76
COD P1 66 454 (137) 190 (76) 60 125 (78) 44 79
P2 68 390 (117) 132 (48) 67 93 (35) 28 76
BOD P1 59 236 (74) 72 (28) 67 49 (34) 42 82
P2 52 226 (74) 59 (15) 78 33 (10) 25 86
TKN P1 60 31 (5) 33 (6) – 23 (8) 40 40
P2 58 32 (4) 37 (4) – 32 (4) 15 15
N-ammonia P1 57 25 (5) 29 (6) – 20 (6) 40 40
P2 66 27 (5) 32 (4) – 27 (5) 15 15
NO2
À
-N P1 60 0.026 (0.074) 0.009 (0.044) – 0.990 (0.832) – –
P2 83 0.033 (0.046) 0.024 (0.026) 1.449 (1.051) – –
NO3
À
-N P1 63 <0.10 <0.10 – 9.16 (4.89) – –
P2 86 <0.10 <0.10 – 2.06 (1.35) – –
Raw sewage: after preliminary treatment; P1 ¼ first phase of operation; P2 ¼ second phase of operation; n ¼ number of samples; ( ) standard deviation.
2749 P. C. Vieira et al. | Performance evaluation of a novel open tricking filter Water Science & Technology | 67.12 | 2013
6. It is seen that the UASB reactor already provides good
removal efficiencies for suspended solids (median of 78
and 73%), COD (median 60 and 67%) and BOD (median
of 67 and 78%), but the OTF gives an additional important
polishing. In terms of TKN and ammonia, the removal
takes place only at the OTF, due to nitrification (as seen
by the nitrate production).
In Table 3 are presented the results of statistical signifi-
cance levels (p) of the non-parametric tests used to compare
the effluent concentrations in the two operational phases.
The results of the statistical tests showed for all parameters,
except for DO, TSS and COD, a significant difference (p <
0.05) between the phases.
Figure 3 shows box-plots of effluent concentrations
compared with discharge standards established by the
state of Minas Gerais, Brazil (DN-CERH COPAM 01/
2008): 100 mg TSS LÀ1
, 180 mg COD LÀ1
, 60 mg BOD LÀ1
,
20 mg N-ammonia LÀ1
.
In terms of suspended solids in the effluent from the
OTF, the compliance with the discharge standard of
100 mg TSS LÀ1
was 95% for phase 1 (P1) and 85% for
phase 2 (P2). Although this standard is somewhat relaxed
compared with international legislation, the compliance
level is considered high for the reality of a developing
country. It should be remembered that there is no second-
ary sedimentation tank in the system, and better results
would probably be obtained if this unit were included in
the treatment line (but of course construction costs would
be higher).
The concentrations of COD and BOD in the effluent
from the OTF led to high compliance levels with the dis-
charge standards of 180 and 60 mg LÀ1
, respectively: COD
86% (phase 1) and 99% (phase 2); BOD 80% (phase 1)
and 98% (phase 2). Again, it should be pointed out that no
secondary sedimentation tanks were included, which
could assist in the removal of particulate organic matter.
Table 3 | Results of p-values from non-parametric tests comparing concentrations in phases of operation of the OTF
Tests DO Alkalinity TSS COD BOD TKN N-ammonia Nitrite Nitrate
Mann–Whitney 0.066 0.000 0.078 0.111 0.001 0.000 0.000 0.027 0.000
Wald–Wolfowitz 0.250 0.000 0.478 0.341 0.000 0.000 0.000 0.713 0.000
Significantly different? No Yes No No Yes Yes Yes Yes Yes
Figure 3 | Box-plot of the concentrations obtained in the monitoring of the UASB–OTF system for each operational phase.
2750 P. C. Vieira et al. | Performance evaluation of a novel open tricking filter Water Science & Technology | 67.12 | 2013
7. The results of N-ammonia concentrations in the OTF
effluent showed a compliance rate of 60% (phase 1: HLRS
4.1 m³ mÀ2
dÀ1
; OLRV 0.08 kg BOD mÀ3
dÀ1
) and only 10%
(phase 2: HLRS 9.3 m³ mÀ2
dÀ1
; OLRV 0.12 kg BOD mÀ3
dÀ1
)
with the discharge standard of 20 mgN-ammonia LÀ1
, with the
OTF presenting an average ammonia removal efficiency of
40% for phase 1 and 15% for phase 2. For the operating con-
ditions of phase 1, this can be considered an advantage of the
investigated system, since most of the simple post-treastment
options for UASB effluents are not able to remove ammonia
(Chernicharo ). In terms of load removal per specific surface
area of the support medium (Figure 4), median values were
higher in phase 2 (0.73 g N-ammonia mÀ2
dÀ1
) compared with
phase 1 (0.60 g N-ammonia mÀ2
dÀ1
), although there was no
significant difference at a 5% significance level.
CONCLUSIONS
The performance of the combined system (UASB þ OTF)
showed satisfactory compliance levels to the regional dis-
charge standards: 95% (phase 1) and 85% (phase 2) for
TSS (standard of 100 mg TSS LÀ1
), 86% (phase 1) and
99% (phase 2) for COD (180 mg COD LÀ1
) and 60%
(phase 1) and 98% (phase 2) for BOD (60 mg BOD LÀ1
).
An important point is related to the performance of the
OTF on the partial removal of ammonia (mean removal effi-
ciency of 40% – obtained in the first operation phase, with
4.3 m3
mÀ2
dÀ1
and 0.08 kg BOD mÀ3
dÀ1
). The concen-
tration of N-ammonia obtained in the first operation phase
in the OTF effluent was associated with a compliance level
of 60% with the standard of 20 mg NH4
þ
-N LÀ1
. It is con-
sidered that this percentage of compliance is good, taking
into account the reality of a developing country, and the dif-
ficulty of removing ammonia in various other biological
wastewater treatment processes. The removal efficiency of
ammonia was probably related to the larger than usual
height of the OTF (leading to a larger reaction volume)
and the absence of side walls (possibly leading to higher
aeration), and the good performance of the UASB reactor
in removing organic matter (COD and BOD).
With the purpose of adopting wastewater treatment
technologies that meet the requirements of operational sim-
plicity, no mechanization, small footprint and satisfactory
performance, the combined system of UASB reactor and
open TF showed to be a promising alternative, especially
for locations with few economical resources.
ACKNOWLEDGEMENTS
The authors thank the support given by these institutions:
Conselho Nacional de Desenvolvimento Científico e Tecno-
lógico – CNPq, Fundação de Amparo à Pesquisa do Estado
de Minas Gerais – FAPEMIG, Fundação Nacional de
Saúde – FUNASA, FINEP – Agência Financiadora de Estu-
dos e Projetos, and Companhia de Saneamento de Minas
Gerais – COPASA. The authors also thank Mr Raimundo
Magela (CePTS operator) for the support during the field
work.
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2752 P. C. Vieira et al. | Performance evaluation of a novel open tricking filter Water Science & Technology | 67.12 | 2013
