Abstract

On-site sanitation systems in Europe are evaluated through a CE marked procedure done on a platform test under a specific schedule of loads. Nevertheless, the test procedure conditions do not represent the real conditions of treatment systems in terms of wastewater characteristics and loads. On another angle, in France, systems implemented for capacities above 20 p.e. do not need the CE marked procedure but have to comply with performance requirements. French on-site treatment regulations lead to a paradoxical situation where constructed wetlands (CW) designed for 21 p.e. can be more compact than for 15 p.e. Here we focus on a single-stage vertical flow CW treating raw wastewater from a six-person house. Working with a (compact) community CW design, the objectives were to evaluate, in real-world conditions, the limits of the system and its ability to handle the high hydraulic and organic load variations found in on-site sanitation. Concentrations and fluxes showed high inter-day and intra-day variability, confirming the necessity for treatment systems to be robust enough for on-site sanitation. The compact CW appeared very efficient and stable for organic pollutants and nitrification (average removal rates of more than 98%, 99%, 94% and 97% for TSS, BOD5, COD and TKN, respectively). Denitrification has been optimized to reach 70% of TN removal, but seems unable to go higher due to a lack of carbon.

INTRODUCTION

On-site sanitation systems in Europe are evaluated through an EU-labeled procedure done on a test platform under a specific loading schedule (NF EN 12566-3 + A1 2009 – ‘Small wastewater treatment systems for up to 50 PT’), but these test procedure conditions are not representative of real-world treatment-system conditions in terms of wastewater characteristics and loads. Studies on household pollutant outputs often find higher concentrations and variable characteristics (Matulova et al. 2010). In some counties, specific national guidelines were established before the EU-labeled procedure to implement constructed wetlands (CW) for on-site sanitation, like in Denmark (Brix & Arias 2005), Austria (Österreichisches Normeringsinstitut 1997) or Germany (revised DWA-A 262 German constructed wetland guideline – Nowak et al. 2018), for example. These configurations can lead to different required filter areas from 3 to 5 m2/p.e. and always implement a primary treatment like a septic tank. However, in France, constructed wetland systems fed with raw wastewaters have to pass the labeled procedure for capacities up to 20 p.e. while, for capacities above 20 p.e., they only have to comply with performance requirements. For CW, this leads to a situation where a 21-p.e. design can be more compact than a 15-p.e. design due to larger labelled designs for on-site sanitation. Some companies offer systems that pass the EU-labeled procedure for a single-stage vertical-flow CW (VFCW) receiving raw wastewater with designs of 1.2–2 m2 per inhabitant. These designs concern VFCW that are unsaturated or combined with horizontal-flow CW (HFCW), resulting in either possible solids release according to load variation (VFCW) or a high footprint of 5 m2/p.e. (HFCW). The trade-off between system compactness and system reliability is crucial in the response to high load variations. Design efforts have recently turned to developing compact VFCW to treat raw wastewater from municipalities using recirculation (Prost-Boucle & Molle 2012) or unsaturated/saturated VFCW systems (Prigent et al. 2013; Silveira et al. 2015; Morvannou et al. 2017), but always with fairly regular feeding from municipalities. Nevertheless, biological communities vary (in quality and quantity) in response to feeding/resting period strategies or high load variations (weekdays, holidays, weekends), thus affecting treatment performances, as studied during start-up periods with regular loads (Weber & Legge 2011). Hydraulic load variation can also induce different retention times within the system, again affecting performances. Implementing a saturated vertical layer at the bottom can increase the retention time while promoting the denitrification process and, consequently, consumption of carbon. This type of design may appear more robust in buffering outlet quality than totally unsaturated VFCW systems.

The aim of this study was to evaluate, in real-world conditions, the behavior of a single-stage unsaturated/saturated VFCW (as designed for small communities) fed by raw wastewater (with no septic tank). The objectives were to evaluate system performances, robustness and resilience in response to high load variations from households, and to optimize the unsaturated layer depth.

MATERIAL AND METHODS

Monitoring was conducted on an unsaturated/saturated VFCW treating the wastewater from a six-person family in the south of France (Crest, Drôme department). The system consists of:

  • A pumping station receiving all wastewaters from the house (no screening) and delivering, on average, four or five batches a day to the treatment system. Batch feeding flow is 3 m3/h for an average batch volume of 80 liters (so each batch lasts around 1.5 minutes). Batches are applied on one filter surface by one feeding point.

  • An unsaturated/saturated VFCW divided into two parallel filters to implement one-week feeding/one-week resting cycles. The filters (1.2 m deep) comprise a 70 cm layer of 2/6 mm crushed-washed gravel (unsaturated layer), a 10 cm transition layer (6/12 mm) with an aeration pipe inside (0.3 m/m2), and a saturation layer composed of pea gravel (20/40 mm). We tested saturation layer depths between 22 and 40 cm to optimize denitrification. Each filter has a surface of 7 m2 (3.5 m × 2 m). Filter borders have a 1-to-1 slope and the filters are sealed thanks to a polypropylene geomembrane.

Treated wastewaters were infiltrated into the ground in winter and reused for irrigation of bushes the rest of the year.

Monitoring started after 3 years of operation and was continued for 18 months, and consisted of:

  • Counting the number of people present in the house overnight and at lunch and dinner every day.

  • Measuring daily flow via a pressure probe (STS) in the pumping station recording measurements at one-minute time-steps.

  • Performing 24 h flow proportional samples at the inlet/outlet of the system to analyze global parameters by standard methods (APHA 2012) for chemical oxygen demand (COD), dissolved COD, total organic carbon (TOC), dissolved organic carbon (DOC), total suspended solids (TSS), biochemical oxygen demand (BOD5), total Kjeldahl nitrogen (TKN), N-NH4+, N-NO2, N-NO3, P-PO43− and total phosphorus (TP). A total of 31 campaigns were done in different seasons. Samplings were performed over 7 to 10 consecutive days to analyze the dynamics of treatment performances. Total nitrogen (TN) was estimated by the sum of TKN and N-Nox.

  • For some campaigns, N-NO3/N-NH4 were measured online at the outlet at one-minute time-steps (Ion Selective Electrode sensor, WTW Varion).

Sampling raw wastewater with high solids concentrations is a tricky exercise, so we were particularly careful with the inlet sampling method. Inlet wastewater sampling was done using a water detector inserted in the feeding pipe at the filter surface. During a batch, the water arriving at the detector made the sampler work for a specific volume, which made the samplings proportional to batches (and thus to flow) and perfectly representative of the wastewater arriving on the filter.

RESULTS AND DISCUSSION

Characterization of per-inhabitant pollutant production

On-site observation of wastewater characteristics found that concentrations were much higher than usually used in EU-labeled tests (see Table 1) and closer to the concentrations observed for on-site sanitation (Cauchi & Vignoles 2012), even from small communities (Butler et al. 1995; Henze 2008; Mercoiret 2010). This reveals that wastewater characteristics from households are highly concentrated and highly variable.

Table 1

Wastewater characteristics

 Volume (L) COD dCOD TSS BOD5 (mg/L) TKN N-NH4 TP 
Average 521 (54 L/cap/day) 1,853 467 1,032 767 209 152 29 
Median 347 1,780 470 1,065 784 207 148 29 
Min 67 358 53 213 530 29 16.8 5.5 
Max 4,397 2,750 708 1,590 1,100 330 252 43.6 
Butler et al. (1995) (average values)     492  22 14 
Cauchi & Vignoles 2012 (average values) 84 L/pe/day    633    
Mercoiret 2010 (average values)  645  288 265 67 55 
Henze (2008) (maximum values) 80 L/cap/day 2,750   1,125 185  35 
EU-labeled tests (range) 150/pe/day 300–1,000  200–700 150–500 25–100 22–80 5–20 
 Volume (L) COD dCOD TSS BOD5 (mg/L) TKN N-NH4 TP 
Average 521 (54 L/cap/day) 1,853 467 1,032 767 209 152 29 
Median 347 1,780 470 1,065 784 207 148 29 
Min 67 358 53 213 530 29 16.8 5.5 
Max 4,397 2,750 708 1,590 1,100 330 252 43.6 
Butler et al. (1995) (average values)     492  22 14 
Cauchi & Vignoles 2012 (average values) 84 L/pe/day    633    
Mercoiret 2010 (average values)  645  288 265 67 55 
Henze (2008) (maximum values) 80 L/cap/day 2,750   1,125 185  35 
EU-labeled tests (range) 150/pe/day 300–1,000  200–700 150–500 25–100 22–80 5–20 

The hydraulic loads received varied significantly, with high loads (maximum of 4.4 m3/d representing a hydraulic load of 63 cm/d on the filter in operation) for some individual events. Excepting these special events (three datapoints), daily volume production and classical within-day variation are shown in Figure 1.

Figure 1

Daily hydraulic load (left) and average hydraulic load distribution during a day (right).

Figure 1

Daily hydraulic load (left) and average hydraulic load distribution during a day (right).

Average daily flow volume was 290 liters per day. Compared to the effective human occupancy each day, daily volume generated per capita was 54 liters on average (Figure 2).

Figure 2

Daily volume generated per capita.

Figure 2

Daily volume generated per capita.

The measurements quantify volume variations that are often doubled, or even tripled or more during special events, from one day to another. The unique special event, with more than 10 times the average hydraulic load, corresponds to an open tap that was forgotten and left running on a day when occupancy was high (the 5th of July 2016). While this event can be seen as an accident and an unusual situation, it is still important to evaluate whether this kind of event can be accepted by the system or damages its functional operation.

To analyze whether volume variations correlated to occupancy variations in the house, we defined the daily average occupancy by cumulating the number of meals (lunch and dinner) plus the number of people who slept overnight in the home, divided by 3. Figure 3 (left) shows a slight correlation between occupancy and emitted volume of domestic wastewater (R2 = 0.26). Volume increased on aggregate with increasing occupancy, but also showed one to six-fold variation for the same occupancy rate. Every day has its specific pattern in terms of wastewater emissions. Figure 3 (right) shows the amplitude and average between open days, weekends and weekdays (the three special events have been removed).

Figure 3

Daily variations in emitted volume as a function of occupancy (left) and day-type (right).

Figure 3

Daily variations in emitted volume as a function of occupancy (left) and day-type (right).

The results obtained for the concentrations and pollutant loads (in carbon, nitrogen and phosphorus) were similar to the emitted volumes (results not shown). Note that even after excluding outliers induced by special events, the treatment system still has to handle huge variations (hydraulic and organic), which underlines how it is vital for real-world treatment systems to be robust enough to maintain treatment performances.

Treatment performances

Based on 60 g of BOD5/p.e./d and 150 g of COD/p.e./d, the system received a corresponding average organic load of about 4 p.e. (one inhabitant is not equal to a population equivalent) with a maximum measured load of 10.5 p.e. Even organic load showed strong and sudden variations (Figure 4).

Figure 4

Organic and hydraulic load variations over consecutive monitoring days for two seasons.

Figure 4

Organic and hydraulic load variations over consecutive monitoring days for two seasons.

Inlet-outlet concentrations for all measured parameters are presented in Table 2. Whatever the inlet concentration variations, outlet concentrations always remained low for carbon pollutant parameters, solids, and ammonia. This demonstrates that the system is stable to inlet hydraulic and organic load variations. For nitrogen removal, inlet/outlet concentrations do not show more variations. There are no obvious differences to observe, according to saturation level, on nitrate outlet concentrations, as inlet TKN concentrations were not similar in the two experimental phases. We will see later that it is clearer on removal performances. Figure 4 charts the organic and hydraulic load variations over consecutive days for two seasons. While hydraulic load remains globally low (excepting the on-off event) compared to what this system is able to accept (Molle et al. 2006), the organic load varies a lot, which could compromise biological stability and efficiency.

Table 2

Inlet–outlet concentrations during 24-h flow-proportional composite sampling campaigns

 TSS
 
COD
 
dCOD
 
BOD5
 
TOC
 
DOC (mg/L)
 
TKN
 
N-NH4
 
N-NO2
 
N-NO3
 
TP
 
P-PO4
 
 in out in out in out in out in out in out in out in out in out in out in out in out 
Average 1,032 10 1,853 62 467 49 767 578 25 163 19 209 12 152 7.8 0.08 0.19 1.4 69 29 17 10 14 
Median 1,065 1,780 57 470 49 784 588 24 157 19 207 148 5.4 0.04 0.16 0.5 62 29 20 15 
Min 213 358 23 53 20 530 435 19 116 14 29 17 0.3 0.02 0.02 0.5 15 
Max 1,590 22 2,750 100 708 82 1,100 16 762 30 240 24 330 29 252 22.5 0.51 0.53 5.1 148 44 23 20 21 
Nb of values 30 30 31 31 31 31 13 13 17 17 13 13 31 22 31 31 31 31 31 31 28 28 28 28 
 22 cm saturation
 
40 cm saturation
 
        
 TKN
 
N-NH4 N-NO2 N-NO3 TKN
 
N-NH4 N-NO2 N-NO3         
 in out in out in out in out in out in out in out in out         
Average 198 11.5 138 8.7 0.09 0.19 1.11 75 239 8.7 193 7.4 0.05 0.14 1.57 68         
Median 195 8.8 134 5.4 0.05 0.16 0.45 57 223 7.7 189 5.8 0.04 0.13 0.55 68         
Min 29 1.7 17 0.8 0.02 0.02 0.45 14.6 188 4.7 151 3.0 0.04 0.09 0.45 60         
Max 294 29.3 235 22.5 0.51 0.53 5.13 148 330 14.2 252 12.7 0.15 0.30 3.2 77         
Nb of values 23 21 23 21 23 21 23 21         
 TSS
 
COD
 
dCOD
 
BOD5
 
TOC
 
DOC (mg/L)
 
TKN
 
N-NH4
 
N-NO2
 
N-NO3
 
TP
 
P-PO4
 
 in out in out in out in out in out in out in out in out in out in out in out in out 
Average 1,032 10 1,853 62 467 49 767 578 25 163 19 209 12 152 7.8 0.08 0.19 1.4 69 29 17 10 14 
Median 1,065 1,780 57 470 49 784 588 24 157 19 207 148 5.4 0.04 0.16 0.5 62 29 20 15 
Min 213 358 23 53 20 530 435 19 116 14 29 17 0.3 0.02 0.02 0.5 15 
Max 1,590 22 2,750 100 708 82 1,100 16 762 30 240 24 330 29 252 22.5 0.51 0.53 5.1 148 44 23 20 21 
Nb of values 30 30 31 31 31 31 13 13 17 17 13 13 31 22 31 31 31 31 31 31 28 28 28 28 
 22 cm saturation
 
40 cm saturation
 
        
 TKN
 
N-NH4 N-NO2 N-NO3 TKN
 
N-NH4 N-NO2 N-NO3         
 in out in out in out in out in out in out in out in out         
Average 198 11.5 138 8.7 0.09 0.19 1.11 75 239 8.7 193 7.4 0.05 0.14 1.57 68         
Median 195 8.8 134 5.4 0.05 0.16 0.45 57 223 7.7 189 5.8 0.04 0.13 0.55 68         
Min 29 1.7 17 0.8 0.02 0.02 0.45 14.6 188 4.7 151 3.0 0.04 0.09 0.45 60         
Max 294 29.3 235 22.5 0.51 0.53 5.13 148 330 14.2 252 12.7 0.15 0.30 3.2 77         
Nb of values 23 21 23 21 23 21 23 21         

Despite these high load variations, removal performances remained extremely stable, as shown in Figure 5, with average removal rates of more than 98%, 99%, 94% and 97% for TSS, BOD5, COD and KN, respectively whatever the saturation level. Even though hydraulic load is usually low, organic load can reach values similar to nominal design of French VFCWs (Molle et al. 2005).

Figure 5

Applied and treated loads on the filter in operation.

Figure 5

Applied and treated loads on the filter in operation.

TN removal was still not complete and there was no clear pattern of seasonal correlation. Increasing the depth of the bottom saturation layer from 22 cm to 40 cm appeared to improve TN removal (bottom right, Figure 5). With 22 cm of saturation, TN removal was about 45% and not stable, while with 40 cm of saturation, TN removal reached around 70%. Around 68 mg N-NO3/L were still present at the outlet. The question of whether the retention time in the saturation zone (around 1.3 days on average when saturation is fixed to 40 cm) or the source of carbon were limiting to reach higher denitrification levels can be discussed. On one hand, such a retention time appears sufficient to reach high denitrification levels when the carbon source is high enough (Morvannou et al. 2017). On the other hand, average and median outlet COD concentrations were 68 and 69 mg COD/L, respectively, when implementing a 22 cm depth of saturation and 46 and 45 mg COD/L, respectively, when implementing a 40 cm depth of saturation. Outlet median BOD5 concentrations were 3.2 mg/l with a saturation layer of 40 cm. Those outlet COD concentrations are close to inert COD as defined by different studies (see Choi et al. 2017 for instance) and anyway not enough to denitrify 60 mg/L of nitrate. It is likely that the carbon source was the limiting factor.

Analysis of intra-day nitrogen outlet quality variation (Figure 6) showed that the system was observably robust. Even with the applied load varying from 2.5 to 19 g·m−2·d−1, the outlet concentration for N-NH4 and N-NO3 remained quite stable.

Figure 6

Online monitoring of the nitrogen formed at the outlet (time-lapse: 1 minute).

Figure 6

Online monitoring of the nitrogen formed at the outlet (time-lapse: 1 minute).

Phosphorus is not correctly treated in the filter as no specific materials are implemented. Only 40% on average is removed, mainly the particulate form of P that is filtered by the system.

Feeding the system with raw wastewater should lead to an accumulation of an organic deposit on the filter surface (Molle et al. 2005). Due to the low load applied, and despite the fact that reeds were pruned but not removed from the filter, there was no measurable deposit layer depth after 5 years in operation. The bulk of the deposit is mineralized, which facilitates sludge management and confirms the value of feeding the system with raw wastewater.

CONCLUSIONS

In real-scale monitoring over 18 months on an on-site constructed wetland system some general conclusions emerged.

In terms of wastewater production and variation for this single-household:

  • Wastewater concentration characteristics are much higher than usually used in EU-labeled tests.

  • There is not a proportional relationship between the number of people in the house and the amount of emitted wastewater. The emitted volume per person is on average three times lower than the reference value used in the EU-labeled tests, but can vary from 4 up to 304 liters per day.

  • Organic load variation can vary from day to day by up to a factor of 10, which means treatment systems are required to be robust and resilient to unexpected events. The studied system showed a decrease in the nitrification rate in Figure 6 with the special event (open tap for one day) due to the lower hydraulic retention time.

In terms of performances of the unsaturated/saturated VFCW tested, we noted that the system remained exceptionally stable whatever the variations observed during monitoring. Removal performances were very high, and outlet concentrations were low and stable despite the big inlet variations. While performances were high for solids, carbon removal and nitrification, performance in TN removal was impacted by the depth of the saturation level. The design implemented here seems unable to achieve better than 70% TN removal as it still lacks a carbon source for denitrification at the outlet. To reach higher levels, the design will have to balance TN removal against nitrification performance.

REFERENCES

REFERENCES
APHA
2012
Standard Methods for the Examination of Water and Wastewater
.
American Public Health Association/American Water Works Association/Water Environment Federation
,
Washington, DC
,
USA
.
Butler
D.
,
Friedler
E.
&
Gatt
K.
1995
Characterising the quantity and the quality of domestic wastewater inflows
.
Water Sci. Technol.
31
(
7
),
13
24
.
Cauchi
A.
&
Vignoles
C.
2012
Caractéristiques des eaux usées brutes de la maison individuelle
.
Eau Industrie et Nuisances
354
,
84
90
.
Henze
M.
,
Comeau
Y.
2008
Wastewater Characterization. In:
Biological Wastewater Treatment: Principles, Modelling and Design
(
Henze
M.
,
van Loosdrecht
M.C.M.
,
Ekama
G. A.
&
Brdjanovic
D.
, eds).
IWA Publishing
,
London
,
UK
.
Matulova
Z.
,
Hlavinek
P.
&
Drtil
M.
2010
One-year operation of single household membrane bioreactor plant
.
Water Sci. Technol.
61
(
1
),
217
226
.
Mercoiret
L.
2010
Qualité des Eaux Usées Domestiques Produites par les Petites Collectivités
. .
Molle
P.
,
Liénard
A.
,
Boutin
C.
,
Merlin
G.
&
Iwema
A.
2005
How to treat raw sewage with constructed wetlands: an overview of the French systems
.
Water Sci. Technol.
51
(
9
),
11
21
.
Morvannou
A.
,
Troesch
S.
,
Esser
D.
,
Forquet
N.
,
Petitjean
A.
&
Molle
P.
2017
Using one filter stage of unsaturated/saturated vertical flow filters for nitrogen removal and footprint reduction of constructed wetlands
.
Water Sci. Technol.
76
(
1
),
124
133
.
NF EN 12566-3 + A1
2009
Small Wastewater Treatment Systems for up to 50 PTE
.
AFNOR publication
.
Nowak
J.
,
van Afferden
M.
,
Albold
A.
,
Bernhard
K.
,
Fehr
G.
,
Galander
C.
,
Hasselbach
R.
,
Heise
B.
,
Kühn
V.
,
Langergraber
G.
,
Molle
P.
,
Nivala
J.
,
Rustige
H.
&
Stockbauer
M.
2018
The new German standard on constructed wetland systems for domestic and municipal wastewater treatment
. In:
16th IWA Specialized Group Conference on Wetland Systems for Water Pollution Control
,
1–4 October 2018
,
Valencia, Spain.
Österreichisches Normeringsinstitut
1997
Bepflanzte Bodenfilter (Pflanzenkläranlagen) Anwendung, Bemessung, Bau und Betrieb
.
ÖNORM B 2505
.