(633f) 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

*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

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

* 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. 

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.