Total Phosphorus Removal from Sewage in Microbial Electrochemically Assisted Septic Tanks

Background

Subsurface sewage treatment systems (SSTS), commonly known as septic systems, typically consist of a septic tank and an absorption field (or drainfield or leachfield), to treat sewage from households. The septic tank as pretreatment step decreases total suspended solids (TSS), biochemical/chemical oxygen demands (BOD/COD), and pathogen levels from sewage influent, while the drainfield or soil absorption field captures nutrients (nitrogen and phosphorus) from the tank effluent so that the water leaving SSTS would have minimal environmental impact. By modifying the conventional septic tank through incorporating electrode materials to make use of microbial electrochemical process, this study assessed this novel design in terms of sewage treatment performance. Since phosphorus becomes increasingly threatening to water environment, it was the goal of this novel design to remove it from liquid phase and potentially capture it at electrode surface.

Methods

The experiments were conducted with 1-L scale simulated septic tanks at four different applied voltage levels (0.50 V, 0.63 V, 0.75 V, and 0.88 V, selection based on previous preliminary results) with control of without applied voltage. Tanks were set up and operated at two temperatures (15 and 25 ºC), a hydraulic retention time of 8.3 days, two cycles of feeding and withdrawal during each HRT. The treatment conditions of the reactors are listed in  REF _Ref450506281 \h Table 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400350030003500300036003200380031000000 . ANOVA and pairwise multiple comparisons based on Bonferroni test were conducted for water characteristics with OriginPro Version 8 at significance level of α=0.05.

Table  SEQ Table \* ARABIC 1. Treatments of simulated novel septic tanks for 171-day operation

	Treatment*
Reactor #	Temp, oC	Voltage, V	Chlortetracycline (CTC)
Reactor #1	25	0	No
Reactor #2	25	0.50	No
Reactor #3	25	0.63	No
Reactor #4	25	0.75	No
Reactor #5	25	0.88	No
Reactor #6	15	0	No
Reactor #7	15	0	1 mg-CTC/L
Reactor #8	15	0.50	No
Reactor #9	15	0.63	No
Reactor #10	15	0.75	No
Reactor #11	15	0.88	No
Reactor #12	15	0.75	1 mg-CTC/L
Reactor #13	15	0.88	1 mg-CTC/L

*Water characteristics data of reactors #6 and #7 were pooled for data analysis, and the same procedure was applied to reactors #10 and #12, and reactors #11 and #13, for the reason that CTC addition was not detected to affect other water characteristics.

Results and Discussion

For total phosphorus, results suggested that septic tanks partially removed it from influent ( REF _Ref450507954 \h Table 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400350030003500300037003900350034000000 ), and the novel design of microbial electrochemical further decreased its concentration (p=0.0001) with an influence from temperature (p=0.0003), while the interaction of voltage and temperature was not significant (p=0.2723). Those performance differences in total phosphorus removal were further elucidated by pairwise multiple comparisons given in  REF _Ref450508201 \h Table 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400350030003500300038003200300031000000 . The results concluded that at 25 ºC, 0.63 V or higher voltage at the selected electrode material is practically effective to remove most total phosphorus (>89.7%) from sewage, resulting in an effluent TP concentration less than 0.82 mg/L over 171-day operation; and that at 15 ºC, phosphorus removal efficiency was smaller and it required 0.88 V to remove over 90% of total phosphorus from sewage.     

Table  SEQ Table \* ARABIC 2. Total phosphorus concentrations in influent and effluents of different reactors

	Ave	SD	TP removal
	mg/L	mg/L	%
Influent	7.95	1.39	-
Control, 25 ºC	6.98	1.72	12.2
0.50 V, 25 ºC	1.81	3.67	77.2
0.63 V, 25 ºC	0.82	2.92	89.7
0.75 V, 25 ºC	0.14	2.30	98.3
0.88 V, 25 ºC	0.11	2.31	98.7
Control, 15 ºC	7.36	1.60	7.45
0.50 V, 15 ºC	6.30	3.84	20.7
0.63 V, 15 ºC	4.31	3.90	45.8
0.75 V, 15 ºC	2.26	2.48	71.6
0.88 V, 15 ºC	0.49	2.36	93.9

Table  SEQ Table \* ARABIC 3. Pairwise multiple comparisons results for total phosphorus levels in effluents

Comparisons (Bonferroni Test)	Significance at alpha=0.05 or lower
0.50 V, 25 ºC  vs. Control, 25 ºC	*
0.63 V, 25 ºC  vs. Control, 25 ºC	*
0.63 V, 25 ºC  vs. 0.50 V, 25 ºC	NS
0.75 V, 25 ºC  vs. Control, 25 ºC	*
0.75 V, 25 ºC  vs. 0.50 V, 25 ºC	*
0.75 V, 25 ºC  vs. 0.63 V, 25 ºC	NS
0.88 V, 25 ºC  vs. Control, 25 ºC	*
0.88 V, 25 ºC  vs. 0.50 V, 25 ºC	*
0.88 V, 25 ºC  vs. 0.63 V, 25 ºC	NS
0.88 V, 25 ºC  vs. 0.75 V, 25 ºC	NS
Control, 15 ºC  vs. Control, 25 ºC	NS
Control, 15 ºC  vs. 0.50 V, 25 ºC	*
Control, 15 ºC  vs. 0.63 V, 25 ºC	*
Control, 15 ºC  vs. 0.75 V, 25 ºC	*
Control, 15 ºC  vs. 0.88 V, 25 ºC	*
0.50 V, 15 ºC  vs. Control, 25 ºC	NS
0.50 V, 15 ºC  vs. 0.50 V, 25 ºC	*
0.50 V, 15 ºC  vs. 0.63 V, 25 ºC	*
0.50 V, 15 ºC  vs. 0.75 V, 25 ºC	*
0.50 V, 15 ºC  vs. 0.88 V, 25 ºC	*
0.50 V, 15 ºC  vs. Control, 15 ºC	NS
0.63 V, 15 ºC  vs. Control, 25 ºC	*
0.63 V, 15 ºC  vs. 0.50 V, 25 ºC	*
0.63 V, 15 ºC  vs. 0.63 V, 25 ºC	*
0.63 V, 15 ºC  vs. 0.75 V, 25 ºC	*
0.63 V, 15 ºC  vs. 0.88 V, 25 ºC	*
0.63 V, 15 ºC  vs. Control, 15 ºC	*
0.63 V, 15 ºC  vs. 0.50 V, 15 ºC	*
0.75 V, 15 ºC  vs. Control, 25 ºC	*
0.75 V, 15 ºC  vs. 0.50 V, 25 ºC	NS
0.75 V, 15 ºC  vs. 0.63 V, 25 ºC	NS
0.75 V, 15 ºC  vs. 0.75 V, 25 ºC	*
0.75 V, 15 ºC  vs. 0.88 V, 25 ºC	*
0.75 V, 15 ºC  vs. Control, 15 ºC	*
0.75 V, 15 ºC  vs. 0.50 V, 15 ºC	*
0.75 V, 15 ºC  vs. 0.63 V, 15 ºC	*
0.88 V, 15 ºC  vs. Control, 25 ºC	*
0.88 V, 15 ºC  vs. 0.50 V, 25 ºC	NS
0.88 V, 15 ºC  vs. 0.63 V, 25 ºC	NS
0.88 V, 15 ºC  vs. 0.75 V, 25 ºC	NS
0.88 V, 15 ºC  vs. 0.88 V, 25 ºC	NS
0.88 V, 15 ºC  vs. Control, 15 ºC	*
0.88 V, 15 ºC  vs. 0.50 V, 15 ºC	*
0.88 V, 15 ºC  vs. 0.63 V, 15 ºC	*
0.88 V, 15 ºC  vs. 0.75 V, 15 ºC	*

* indicates significant difference, p-values less than 0.05; NS indicates non-significant difference, p-values equal or larger than 0.05.

Sludge solids were collected from simulated septic tanks (reactors at 25 ºC,  REF _Ref450512244 \h Figure 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400350030003500310032003200340034000000 ) and quantified for mixed-liquor (ML) total suspended solids (TSS) and total solids (TS), sludge volume index (SVI), and composition. After log transformation, both MLTS and MLTSS displayed a linear increase along with voltage level. At 0.63 V where TP removal achieved close to 90%, the MLTSS and MLTS increased by 2.53 and and 1.17 folds, respectively. However, the corresponding SVI of the sludge was only 52.5% of the control reactor, indicating the formation of denser sludge, which suggested a smaller increase of sludge volume (85.3%) than the weight increase. The composition analysis based on energy dispersive X-ray spectroscopy (EDS) revealed the concentration and amount of phosphorus in sludge samples ( REF _Ref450512615 \h Table 4 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003400350030003500310032003600310035000000 ). More phosphorus was captured in sludge sediment when voltage was increased, but the phosphorus balance, assumed to be captured on electrode surface, was higher at voltages levels of 0.50 and 0.63 V than 0.75 and 0.88 V.

Figure  SEQ Figure \* ARABIC 1. Mixed-liquor total suspended solids (A), mixed-liquor total solids (B), and sludge volume index (C) of the sludge from reactors with different voltage treatments at 25 ºC.

Table  SEQ Table \* ARABIC 4. Phosphorus balance in different reactors. 

	Amt of eff P	Amt P removed	Amt P in Sed	P of balance (on electrodes)
	mg	mg	mg	mg
Inf	178	-	-	-
Control, 25 ºC	143	35	13	21
0.50 V, 25 ºC	37	141	26	114
0.63 V, 25 ºC	17	161	37	125
0.75 V, 25 ºC	3	175	92	83
0.88 V, 25 ºC	2	176	92	83

Conclusions

These results confirmed the phosphorus retention capability of the novel septic tank, suggesting a lower burden on the soil absorption for nutrient removal. The amount of sludge solids was also increased, which may require a more frequent tank cleaning.