### 1. Introduction

_{5}of 90–160 mg/L and fecal coliforms of 10

^{5–}10

^{8}MPN/100 mL [3], higher than the standards for discharge and causing water pollution and high health risk. Main factors affecting the treatment performance of septic tanks were the flow patterns or hydraulic characteristics. The hydraulic conditions have high influence on the treatment performance of reactors and are commonly used to design various reactors. Thus, optimizing reactor configurations and flow rate are important for septic tank designs.

### Objectives

To investigate the hydraulic conditions occurring in on-site sanitation systems and the effect of hydraulic retention times (HRTs) on the system performance.

To evaluate the treatment performance of laboratory-scale and actual-scale septic tanks in term of organic biodegradation and pathogen removals.

To develop a kinetic model for septic tanks for use in design and operation.

### 2. Materials and Methods

### 2.1. Reactor Preparation

### 2.2. Tracer Study

_{t}/C

_{0}according to Levenspiel [4].

*t̄*) was calculated by Eq. (1):

##### (1)

$$\overline{t}=\frac{{\int}_{0}^{\alpha}tCdt}{{\int}_{0}^{\alpha}Cdt}\cong \frac{{\mathit{\Sigma}}_{i}{t}_{i}{c}_{i}\mathit{\Delta}{t}_{i}}{{\mathit{\Sigma}}_{i}{c}_{i}\mathit{\Delta}{t}_{i}}=\frac{V}{v}$$*t*

*is time of first appearance of tracer at the effluent*

_{k}*t̄*and variance, or a measure of the spread of the curve

*σ*

^{2}. The dispersion number 0 means no dispersion or ideal plug flows, while the infinity value indicates complete-mix [5]. The dispersion number was calculated by Eq. (4):

##### (4)

$${\sigma}_{\theta}^{2}=\frac{{\sigma}_{t}^{2}}{{\overline{t}}^{2}}=2\left[\frac{D}{uL}\right]$$*D*is coefficient of axial dispersion (L

^{2}/T).

### 2.3. Performance Study

^{3}day to maintain the HRT value of 24 hr. The reactor temperatures were controlled at 30°C and 40°C by placing the reactor in heated water. The effluent samples were collected and analyzed for physical, chemical and biological characteristics such as TCOD, BOD

_{5}, TS, TVS and fecal coliforms.

### 2.4. Field Survey in Central Thailand

### 3. Results and Discussion

### 3.1. Tracer Study

*C*was effluent concentration,

*C*

_{0}was initial concentration, k

_{T}is reaction rate coefficient at temperature T (°C) according to Eq. (6) and t was retention time or HRT.

### 3.2. Treatment Performance of Laboratory-Scale Septic Tanks

_{5}, TS, TVS and fecal coliforms of the laboratory-scale septic tanks operating at HRT of 24 hr, during steady state conditions are shown in Table 3. The steady-state conditions were assumed to occur when the removal efficiencies of septic tank effluent from the systems were found to be stable for about 1 week. Due to the high organic content of toilet wastewater, the TVS/TS ratio was about 0.9, similar to those values previously reported in the literature [3]. From the experimental period of 4 months, the percent removal of TCOD, BOD

_{5}, TS and TVS in the laboratory-scale septic tanks operating at 30°C were about 69, 67, 62 and 70 percent, respectively. The fecal coliform concentrations were reduced from 1.0×10

^{7}to 6.34×10

^{6}MPN/100 mL when the HRT value was maintained at 1 day. For the 40°C operation, the percent removal of TCOD, BOD

_{5}, TS and TVS were about 76%, 67%, 72% and 77%, respectively. The fecal coliform concentrations were reduced from 1.0×10

^{7}to 2.1×10

^{6}MPN/100 mL when the HRT value was maintained at 1 day. Since the removal mechanism could be mainly due to sedimentation of solids, hence the removal efficiencies of these two laboratory-scale septic tanks were not high and not much different. Accordingly, the k

_{30}for BOD

_{5}removal calculated from Eq. (5) was found to be 2.04 day

^{−1}. The k values for BOD

_{5}removal at other temperatures can be determined from Eq. (6). However, the k40 determined from Eq. (6) could not adequately predict the treatment performance of the laboratory-scale septic tank operating at 40°C. Therefore a modified complete-mix equation was proposed as described in section 3.4.

### 3.3. Field Survey in Central Thailand

_{5}and fecal coliforms were found to be much higher than the Thailand standards for discharge of treated household wastewaters of 20 mg/L and 1000 MPN/100 mL, respectively [8]. These results are similarly to the data of the laboratory-scale septic tanks (Table 3), which revealed unsatisfactory performance and the septic tank effluents need to be further treated. The unsatisfactory results are hypothesized to be due to the hydraulic conditions which caused high degree of short-circuiting as reported earlier, and improper design and operation of these on-site sanitation systems. Moreover, sludge accumulation in the systems also reduced the mean HRTs, resulting in more short-circuiting and increasing the effluent concentrations. It is strongly recommended that post treatment of the septic tank effluents should be done possibly by electrochemical precipitation or nano-disinfection processes which do not required large land area and are cost-effective. More research in this area will be reported later.

### 3.4. Model Development

_{5}removal data obtained from the laboratory-scale and actual-scale septic tanks were plotted in Fig 4. Based on the relationship between C/C0 and HRT (day), a correction factor was added to Eq. (7) to cover the solubilization effects of organic matter in the sludge layer and other related factors. The modified equation is shown below:

^{2}value of 0.90. However, it should be noted that Eq. (7) and (8) are applicable for the septic tank operation at temperatures of 30–40°C, and further validation of these two equations with septic tank data of different temperatures is recommended.

### 3.5. Application of Results

_{5}to meet the effluent discharge standards. Methods to minimize short-circuiting include providing more baffles, better design of inlet and outlet pipes and extending the length/width ratio of the septic tanks. The treatment efficiency could be improved by employing certain enzymes, catalysts or increasing septic tank temperature which should lead to lower effluent BOD

_{5}and pathogen concentrations.

### 4. Conclusions

_{5}and fecal coliforms exceeding the discharge standards. A modified complete-mix model was developed based on the relationship between BOD

_{5}removal efficiency and HRTs which fitted well with the experimental and field-scale data. Methods to improve performance of septic tanks to minimize short-circuiting and increasing BOD

_{5}and pathogen reduction efficiencies were proposed.