DOMESTIC EFFLUENT TREATMENT THROUGH INTEGRATED SYSTEM OF SEPTIC TANK AND ROOT ZONE Luiz S. Philippi, Rejane H. R. da Costa and Pablo H. Sezerino Departamento de Engenharia Sanitária e Ambiental, Universidade Federal de Santa Catarina, Caixa Postal 476, 88040-900 Florianópolis-SC, Brasil
ABSTRACT According to national statistical data, only 10% of the Brazilian urban population have their sewage treated. In the rural areas, where people usually treat sewage trough tank septic system, this value is not great than 5%. This situation, therefore, depicts a lacking of basic sanitation in Brazil, which, in turns, is responsible for the utilisation of individual system for the treatment of sewage by more than 100 million people. Generally, soils and water rivers are, no longer, the last fate for the discharged effluents. Wetland system for the treatment of domestic sewage have been employed in different situations and arrangements (combined system) always showing outstanding performances. The reasons which qualify these system for the treatment of effluents have been attributed to its low cost, easy maintenance and operation. The experiment was carried out in the Agriculture Secretary’s Training Center from the Santa Catarina State, responsible for attending approximately 66 people daily, and was fed with local effluent. This work assesses the efficacy of such a kind of system, which is composed of a septic tank followed by the root zone, in the treatment of liquid effluents.
KEYWORDS Constructed wetland; pollution control; root zone; rural sanitation; septic tank; wastewater treatment; wetland System; INTRODUCTION The situation of the Brazilian basic sanitation depicts a picture of the social and economic conditions of its population. According to data from the Human Development report (IPEA, 1996) only 10 % of the Brazilian urban population have their sewage treated. This situation reveals, on the other side, that more than 100 million people treat their sewage through the utilisation of septic tanks as individual treatment systems, being soils and water rivers as the last fate for the discharged effluents (Philippi, 1997). Another studies have demonstrated similar situation in the rural areas where only 2 % of the total residences have available sewage treatment services while another 5 % disposes their sewage in septic tanks (Lobo e Santos, 1993).These data point up the lacking of basic sanitation in Brazil, which, in turns, justify the application of new technologies in this field. Therefore, this works presents a well known and world-wide utilised system (primary treatment- septic tank) associated with a secondary treatment (root zone) that is totally supported by the soil-plant relationship. From the environmental point of view, a sanitation design should be able to respond for
sanitary questions that takes into account both the quality of the considered environment and the sources of its disturbs. In this way, a well planned design should not only to attempt on a qualitatively and quantitative management of the effluents, but also to preserve the quality of the acceptation points as well as its different uses. The literature have summarised the information concerned the criteria for the selection of sewage treatment systems. Phillipi (1997), presents some authors that have enlightened the different applications and associations of such systems, including those ones called wetlands ( Melo, 1978; Terada, 1985; Bernardes, 1986; Hammer, 1989; Conte et al., 1992; Netter, 1993; Boutin et al.,1994; Crabtree e Rowel, 1994; von Sperling, 1994; Amorim et al., 1997). Recently, two Brazilian scientific publications focused on the use of soil-plant relationships in the soil disposal systems as well as in the surface run-off. In spite of its vast literature, sewage disposal systems in soils demands long term researches. Therefore, in order to validate these technologies, it would be necessary to invest in long term programs. Among the reported features on the utilisation of soil disposal systems or phytopedhological processes are included : the effluent hydraulic distribution on the surface, the soil moisture content, the relationship between humidity and plant species, the colmatation principles of the treatment and the effluent adjustment to plant species. This work presents operational results from a sewage treatment unity, called root zone, that accepts effluents provided by a septic tank. METHODS
Experimentation site The experimental set was built in the Training Centre located in the Agronomica municipal district (latitude 27o 14’15’’S and longitude 49o 38’36’’W) and belongs to the Santa Catarina State’s Research and Technology Company (EPAGRI). The system comprises a septic tank followed by a root zone and was initially set on 13/01/94 (figure 1). The effluents, provided by a craftsman unity from the Training Centre, were: cheese serum, fat, blood, canned food, inbedded swine meat and sanitary sewage arisen from approximately 66 work people. Point 2
Point 3
Point 1 sewage Inlet Flowing Control Box
Septic Tank
Root Zone
Outlet Flowing Control Box
Figure 1. Design of the system established at Agronômica/SC Treatment System Structure
Treated effluent
Before reaching the septic tank, the sewage is driven to a flowing gauge and a fat box. The septic tank was built according to Brazilian Association of Technical Rules, NBR 7229 (ABNT, 1993). It is constituted of masonry with a divisor wall in its inner side. The dimensions of the tank are: 4,00 m total length (2,80 m 1st room; 1,20 m 2nd room), 2,00 m width, 1,70 m useful height and 2,10 m total height. The septic tank has an useful volume of 13,6 m3.After passing through the septic tank, the effluent is driven to the root zone. Its dimensions are: 30,00 m total length, 15,00 m width and 0,70 m depth. The filling material is composed of rough sand, rice rind and grit. Layers (0,10 m tickness) were disposed in an alternated form for sand and grit in the first 0,30 m height. Afterwards, these layers were reduced to 0,05 m tickness. Rice rinds, which help to start the process, were laied before and after setting the sand and grit layers. The trench was covered with impermeable cloth being its edges filled with no 2 rock. The inlet and outlet drains were also covered with no 2 rock (figure 2). Zizaniopsis bonariensis was the plant species selected for this work, in spite of being it under botanical classification. Similarly to juncus, this grass can be easily found in swampy zones from South Brazilian Regions. Cuttings of this species were set up in the root zone tank at 0,30 m spaces.
Figure 2. Simplified view of the root zone trench Assessment and analysis of the sewage samples Three points were chosen for the sample collection: inlet (point 1) and outlet ( point 2) septic tank and outlet root zone (point 3). The collections were performed monthly between july 1994 and june 1995. The analytical parameters assessed in this study were: pH, Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD5), Total Solids (TS- fixed and volatile), Total Suspended Solids (TSS- fixed and volatile), Total Nitrogen or Kjedhal (TN), Total Phosphurus (TP) and Nitrates. The analysis, carried out under the prescriptions of the Standard Methods for the Examination of Water and Wastewaters , were done at the Brazilian Company for Research Farming laboratory (EMBRAPA) located in the City of Concordia/SC. The BOD5 and phosphorus lectures were performed during a 9 month period. RESULTS AND DISCUSSION
It is important to emphasise the process undertaken at the root zone only achieves its plenitude when juncus reaches the maturity phase. Another important aspect concerns the anaerobic digestion starting point at the septic tank. According to Phillipi (1992) this
process only reaches the methane gas phase after an operation time of 2 years. However, the results shown in this work are related to an operation time preceding both the maturity and methane gas phases, respectively, for the root zone and the septic tank. The average values achieved at the sample collection and related to the parameters under study are shown in the table 1. Table 1. Average values and standard deviation achieved upon 12 observation periods for the root zone system located at Agronômica/SC % de Removed
Sampled Points
in relation to point 1
Observed Point 2
Point 3
affter ST
affter RZ
6,6±0,2
6,2±0,4
6,6±0,2
____
____
COD (mg/L)
1045±222
695±196
302±143
33
71
BOD (mg/L)
449±75
306±49
138±62
32
69
Total Solids (mg/L)
1083±180
771±90
427±164
29
61
Volatile Solids (mg/L)
673±156
434±40
168±93
36
75
Fixed Solids (mg/L)
405±96
337±77
244±124
17
45
Total Suspended Solids (mg/L)
119±32
180±46
74±25
____
38
Fixed Suspended Solids (mg/L)
35±16
58±29
38±16
____
____
Volatile Suspended Solids (mg/L)
84±19
122±50
36±18
____
57
Total Nitrogen (mg/L)
224±73
212±61
50±21
5
78
Total Phosphorus (mg/L)
47±18
41±17
13±3
13
72
Nitrates (mg/L)
10±0,9
6±1,7
2±2,5
40
80
Parameters pH
Point 1 in natura sewage
Point 2 Point 3
The achieved average values for the in natura sewage were considerably higher than those ones found for sewage from domestic origin. This fact was due, possibly, to the industrial sewage contribution. Concerning the organic material, were observed values that ranged between 250-350 mg/L for BOD5 and between 450-800 mg/L for COD. For Total Solids (TS), the effluent concentration is on average for domestic sewage while for Total Suspended Solids (TSS) the raw effluent concentration presented a relatively small value, 119 mg/L against 400 mg/L as found in the literature (von Sperling, 1995). The concentration found for the effluent at the septic tank pointed up to a normal value for TSS (180 mg/L) as well as for COD and BOD5 , although the last parameter had presented a higher value than the one found in the literature. The figure 3 points up to a trend for COD stabilisation in the outlet’s treatment system (point 3) and Figure 4 shows the Total Suspended Solids evolution during the sample period.
1800 1600
COD (mg/L)
1400 1200
Point 1
1000
Point 2
800
Point 3
600 400 200 Jun/95
may
april
march
february
january
december
november
october
september
august
Jul/94
0
Period
Figure 3. COD evolution for the sampled points
300
Total SS (mg/L)
250 200
Point 1 Point 2
150
Point 3 100 50
Jun/95
may
april
march
february
january
december
november
october
september
august
Jul/94
0
Period
Figure 4. Total Suspended Solids (TSS) evolution for the sampled points The concentration found for Total Suspended Solids at the septic tank presented values sometimes higher than those ones found for the inlet’s treatment system or, even, for the raw sewage. This fact can be explained by the difficulty faced during the sample collection time or, equally, by the low solids retention in the inner part of the tank, which is attributed to system’s inlet/outlet devices. The two basic functions of the septic tank, i.e., solids retention and sludge digestion, are strongly related to its dimensions. Anaerobic steps are also involved in this context and can be readily verified by the low sludge accumulation. Another studies, carried out with two 5 m3 septic tanks, pointed up to a sludge height value between 0,5 and 0,6 m after an operation time of 550 days (Phillipi,1992). In this
work, however, was found a sludge height value of 1,17 m after an operation time of 700 days. Without extrapolations, this value reflects the tank conditions during the experimentation time. The emphasis attributed to the septic tank could be justified by the fact that this primary treatment prepares the effluent for its further step in the root zone. Organic material removal in the septic tank showed efficiency rates of 33 % for COD and 32 % for BOD5 while at the root zone the efficiency rates for these parameters increased to 71 and 69 %, respectively. Nutrient removal at the system showed efficiency rates of 78 % for Total Nitrogen (TN ) and 72 % for Total phosphurus (TP). The low concentration of these elements observed in the system’s outlet demonstrated, additionally, the occurrence of denitrification processes at the root zone. Figures 5 and 6 shows, respectively, Total Nitrogen and Total Phosphurus evolution for the sample period. Both figures show a stabilisation trend, after the 4th month, for TNK and TP parameters.
450
350 300
Point 1
250
Point 2
200
Point 3
150 100 50
Jun/95
may
april
march
february
january
december
november
october
september
august
0
Jul/94
Total Nitrogen (mg/L)
400
Period
Figure 5. Total Nitrogen (TN) evolution for the three sampled points
Total Phosphurus (mg/L)
80 70 60 50
Point 1
40
Point 2 Point 3
30 20 10 Jun/95
may
april
march
february
january
december
november
october
september
august
Jul/94
0
Period
Figure 6. Total Phosphurus (TP) evolution for the three sampled points CONCLUSIONS
The results achieved for the integrated system of septic tank and root zone allowed the following conclusions: -
For carbonaceous pollution were observed notable removal efficiencies for both COD and BOD5 parameters. In terms of nutrient removal (N and P) were observed efficiency rates above 70 %. Low concentration of nitrates were observed at the system’s outlet. This fact points up to the existence of denitrification processes at the root zone. Higher concentrations of Total Suspended Solids at the system’s outlet than those one observed for plant biomass conversions at the root zone. Easy operation and maintenance for this treatment system.
The treatment system, in spite of needing additional time to reaches its plenitude (2 years after the implantation time), produced a final effluent that fitted well on the standards required by the Environmental Law of the Santa Catarina State. Therefore, the utilisation of a integrated system of septic tank and root zone behaved as a viable technology for the treatment of domestic and industrial effluents, which justifies the investment of more detailed studies, as well as long term programs, in this field. ACKNOWLEDGMENTS
The authors wish to tank “Empresa de Pesquisa e Difusão Agropecuária de Santa Catarina” and Fábio Manhões for assistance in chemical analyses.
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