Abstract

The energy content of screenings from six municipal wastewater treatment plants (WWTPs) was examined. Hourly samples of separated screenings were taken over 24 hours at three of the plants to illustrate diurnal variations. To recover the chemical energy, which usually leaves the WWTP with the screenings, a screenings wash press was used to transfer organic matter from the solid into the liquid phase. The chemical energy of raw and compacted washed screenings as well as the chemical energy of washing water were determined by measuring the chemical oxygen demand (COD) for the six WWTPs. A mass weighted average of 1.35 gCOD/gdm (dm: dry matter) was found in the raw screenings of three WWTPs. The overall recovered energy from screenings was found to range from 0.27 to 0.62 gCOD/gdm. This washed-out COD found in the washing water could be sent for anaerobic digestion or to the wastewater treatment process as a carbon source for denitrification.

INTRODUCTION

Energy efficiency plays an important role in wastewater treatment. Electricity consumption of Austrian wastewater treatment plants (WWTPs) including nutrient elimination ranges from 20 to 30 kWhel per population equivalent (PE) and year for large plants (>100,000 PE; Lindtner et al. 2008). However, municipal wastewater itself also contains a lot of energy – chemical, thermal and potential energy. The recovery of chemical energy should be enhanced in order to improve the energy efficiency of WWTPs. To this end, as many organic compounds as possible should be transferred to digesters as long as the nitrogen removal is not affected by carbon limitation.

The chemical energy in wastewater can be expressed as chemical oxygen demand (COD). The range for the daily COD load emitted per inhabitant found in the literature is shown in Table 1.

Table 1

COD load in municipal wastewater per inhabitant and day in different countries (M: Mean; P: 85th percentile)

Authors/reference   COD load 
Country gCOD/(PE*d) 
Almeida et al. (1999)  UK 112 (M) 
Andreottola et al. (1994)  Italy 116 (M, rural) – 120 (M, urban) 
Henze (1997)  Denmark 130 (M) 
Jönsson et al. (2005)  Sweden 134 (M) 
Schmidt et al. (2003)  Germany 108 (M)/143 (P) 
Svardal & Kroiss (2011)  Austria 110 (M) 
Teichgräber et al. (2014)  Germany 130 (M)/175 (P) 
Authors/reference   COD load 
Country gCOD/(PE*d) 
Almeida et al. (1999)  UK 112 (M) 
Andreottola et al. (1994)  Italy 116 (M, rural) – 120 (M, urban) 
Henze (1997)  Denmark 130 (M) 
Jönsson et al. (2005)  Sweden 134 (M) 
Schmidt et al. (2003)  Germany 108 (M)/143 (P) 
Svardal & Kroiss (2011)  Austria 110 (M) 
Teichgräber et al. (2014)  Germany 130 (M)/175 (P) 

Using the average of the M-values from Table 1 (120 gCOD/(PE*d)) and assuming that 1 kg COD contains 3.5 kWh (Olsson 2015), the chemical energy entering a WWTP can be given a value of 153 kWh/(PE*a). This value is more than four times the electrical energy needed to treat wastewater under the assumption that there are no energy losses during the conversion of one energy type to another. Currently, however, most WWTPs need external energy to achieve the required effluent quality.

Screenings from WWTP have not yet received much attention in terms of their energy content. The major focus of screenings treatment so far has been to reduce disposal costs by compacting them. However, the organic fraction of screenings can partly be recovered by washing. This not only leads to a lower weight of the treated screenings but also improves the energy efficiency of the overall WWTP process because the recovered organic fraction can be converted into methane by digestion.

Very few international publications deal with the amount and characterisation of screenings from WWTPs (Le Hyaric et al. 2009). German publications were therefore included (Röper & Klauke 1981; Branner 2013; Kuhn 2014). The investigations presented in this article show detailed results about the hourly mass flow of screenings and the variations in their energy content over one day for three different WWTPs in dry weather conditions. Additionally, the results the washing of screenings are shown together with calculations on the potential of energy recovery from raw screenings. These values are then compared to the influent COD load of the entire WWTP.

MATERIAL AND METHODS

Diurnal variations of screenings were examined during dry weather conditions at three municipal WWTPs (A: ∼ 17,000 PE, B: ∼ 28,500 PE, C: ∼ 46,600 PE) equipped with a 6 mm rake bar screen (WWTP A) or 6 mm step screens (WWTPs B and C) respectively. These WWTPs are connected to combined sewer systems. Wastewater flows mostly by gravity to the WWTP so that little disintegration of solids takes place.

Hourly samples of the raw screenings were taken at each plant over a 24 hour period and weighed. As the composition of these samples was found to be heterogeneous – consisting mainly of toilet paper and other hygiene products, faeces, kitchen residue and plastics – each sample of raw screenings was homogenised with a shovel until a visually homogeneous sample was achieved. After homogenisation, a representative sub-sample was taken for analysis. Investigated parameters of raw screenings were (i) COD of the solid phase to estimate the chemical energy of screenings (unit: mg COD per g dry matter) and (ii) dry matter. Four grab samples of both faeces and toilet paper were also analysed for WWTP B.

After sampling, the raw screenings were washed batchwise in a full-scale screenings wash press (wash press prototype from Huber SE, type WAP SL). The screenings wash press was operated in a batchwise mode with the ratio 9:1 (477 litres of wash water: 53 kg of raw screenings). An impeller drove wash water into the wash tank and caused the turbulence required to wash off organic substances from the screenings. The duration of the washing process was 5 minutes. Preliminary studies had shown that most of the elution occurred during the first minute of the washing process. For the purpose of this study the duration was extended to 5 minutes to ensure that all faecal components were transferred to the wash water. The resulting wash water passed through a 5 mm perforated metal sheet. Particles remaining on the sieve were compacted and disposed off as treated, pressed screenings. Sieved wash water was analysed for COD (at least double determination). The efficiency of the screenings wash process was calculated by dividing the eluted COD measured in the wash water by the COD of the raw screenings. It describes the transfer of the chemical energy of raw screenings into wash water and is used to assess the energy recovery from raw screenings.

The electrical energy needed to drive the screenings wash press was measured during the washing process. The specific recovery of electrical energy was calculated according to the following formula:  
formula
  Selected value 
Erec Specific recovery of electrical energy from washing of screenings [kWhel/kgdm  
rspec Specific recovery of chemical energy [kgCOD/kgdm0.3 (results found in this study) 
cgas Conversion of COD from substrate into biogas during digestion [%] 50 
ηel Electrical efficiency for a combined heat and power plant [kWhel/kWh] 0.35 
cCOD,kWh Conversion factor of COD into kWh [kWh/kgCOD3.5 
  Selected value 
Erec Specific recovery of electrical energy from washing of screenings [kWhel/kgdm  
rspec Specific recovery of chemical energy [kgCOD/kgdm0.3 (results found in this study) 
cgas Conversion of COD from substrate into biogas during digestion [%] 50 
ηel Electrical efficiency for a combined heat and power plant [kWhel/kWh] 0.35 
cCOD,kWh Conversion factor of COD into kWh [kWh/kgCOD3.5 

COD in the liquid phase was measured according to DIN 38 409-H41 (DIN 1980), COD of the screenings dry mass according to DIN 38414-9 (DIN 1986). Dry matter content and loss on ignition were analysed using DIN EN 12880 (DIN EN 2001a) and DIN EN 12879 (DIN EN 2001b) respectively.

To extend the scope of this study, raw screenings from three additional WWTPs were examined. WWTP D (51,000 PE) is equipped with a 5 mm drum screen, WWTP E (437,000 PE) with a 35 mm coarse screen and a 5 mm fine screen, while WWTP F (9,500 PE) uses a 6 mm step screen. The screenings from D (106 kg), E (53 kg) and F (53 kg) were taken and transported to the screenings wash press. The wash process for these screenings took place under the same conditions (ratio 9:1; duration 5 minutes) as described above. Faeces from WWTP E were collected from the fine screen, mixed and analysed in order to confirm the results obtained from the grab samples taken from WWTP B.

RESULTS AND DISCUSSION

Diurnal variations of screenings

The mass weighted average dry matter contents of raw screenings (and loss on ignition) were found to be 17.0% (88.7%), 14.1% (92.1%) and 15.6% (93.7%) for WWTPs A, B and C, respectively. Table 2 shows the results for dry matter and loss on ignition analysis for each hourly sample. It was found that the dry matter content showed variations meaning that chemical energy was related to the dry matter content in the following results.

Table 2

Results for dry matter (DM) content and loss on ignition (LOI) for hourly samples (not mass weighted)

  WWTP A
 
WWTP B
 
WWTP C
 
Hour DM [g/kg] LOI [%] DM [g/kg] LOI [%] DM [g/kg] LOI [%] 
0–1 a.m. 212.0 90.1 143.0 94.4 139.0 93.3 
1–2 a.m. 137.0 93.8 147.0 93.8 126.0 94.7 
2–3 a.m. 159.0 88.3 127.0 93.7 202.0 87.9 
3–4 a.m. 132.0 94.4 114.0 94.5 202.0 87.9 
4–5 a.m. 172.0 89.0 196.0 92.6 139.0 93.3 
5–6 a.m. 142.0 89.7 138.0 92.7 152.0 95.7 
6–7 a.m. 160.0 83.2 137.0 92.0 139.0 93.3 
7–8 a.m. 144.0 90.0 125.0 93.7 147.0 95.2 
8–9 a.m. 187.0 87.7 140.0 94.1 170.0 95.3 
9–10 a.m. 193.0 87.4 144.0 92.9 126.0 94.7 
10–11 a.m. 250.0 81.6 140.0 91.5 170.0 95.3 
11–12 a.m. 156.0 88.1 163.0 91.9 126.0 94.7 
12–1 p.m. 132.0 89.7 124.0 94.5 202.0 87.9 
1–2 p.m. 156.0 91.0 155.0 95.3 202.0 87.9 
2–3 p.m. 132.0 92.1 149.0 92.9 202.0 87.9 
3–4 p.m. 234.0 89.4 140.0 92.3 152.0 95.7 
4–5 p.m. 151.0 89.5 166.0 81.1 152.0 95.7 
5–6 p.m. 119.0 93.0 129.0 89.6 152.0 95.7 
6–7 p.m. 146.0 90.7 124.0 91.3 147.0 95.2 
7–8 p.m. 125.0 94.9 129.0 92.9 147.0 95.2 
8–9 p.m. 130.0 94.5 145.0 88.2 139.0 93.3 
9–10 p.m. 118.0 93.4 155.0 93.2 139.0 93.3 
10–11 p.m. 126.0 93.1 145.0 94.9 152.0 95.7 
11–12 p.m. 171.0 86.8 116.0 94.7 139.0 93.3 
Average 157.7 90.1 141.3 92.4 156.8 93.3 
Standard deviation 34.6 3.3 17.5 2.9 25.6 2.9 
  WWTP A
 
WWTP B
 
WWTP C
 
Hour DM [g/kg] LOI [%] DM [g/kg] LOI [%] DM [g/kg] LOI [%] 
0–1 a.m. 212.0 90.1 143.0 94.4 139.0 93.3 
1–2 a.m. 137.0 93.8 147.0 93.8 126.0 94.7 
2–3 a.m. 159.0 88.3 127.0 93.7 202.0 87.9 
3–4 a.m. 132.0 94.4 114.0 94.5 202.0 87.9 
4–5 a.m. 172.0 89.0 196.0 92.6 139.0 93.3 
5–6 a.m. 142.0 89.7 138.0 92.7 152.0 95.7 
6–7 a.m. 160.0 83.2 137.0 92.0 139.0 93.3 
7–8 a.m. 144.0 90.0 125.0 93.7 147.0 95.2 
8–9 a.m. 187.0 87.7 140.0 94.1 170.0 95.3 
9–10 a.m. 193.0 87.4 144.0 92.9 126.0 94.7 
10–11 a.m. 250.0 81.6 140.0 91.5 170.0 95.3 
11–12 a.m. 156.0 88.1 163.0 91.9 126.0 94.7 
12–1 p.m. 132.0 89.7 124.0 94.5 202.0 87.9 
1–2 p.m. 156.0 91.0 155.0 95.3 202.0 87.9 
2–3 p.m. 132.0 92.1 149.0 92.9 202.0 87.9 
3–4 p.m. 234.0 89.4 140.0 92.3 152.0 95.7 
4–5 p.m. 151.0 89.5 166.0 81.1 152.0 95.7 
5–6 p.m. 119.0 93.0 129.0 89.6 152.0 95.7 
6–7 p.m. 146.0 90.7 124.0 91.3 147.0 95.2 
7–8 p.m. 125.0 94.9 129.0 92.9 147.0 95.2 
8–9 p.m. 130.0 94.5 145.0 88.2 139.0 93.3 
9–10 p.m. 118.0 93.4 155.0 93.2 139.0 93.3 
10–11 p.m. 126.0 93.1 145.0 94.9 152.0 95.7 
11–12 p.m. 171.0 86.8 116.0 94.7 139.0 93.3 
Average 157.7 90.1 141.3 92.4 156.8 93.3 
Standard deviation 34.6 3.3 17.5 2.9 25.6 2.9 

The total amount (water and dry mass) of raw screenings over 24 hours was 275 kg (WWTP A), 224 kg (WWTP B) and 326 kg (WWTP C). An extrapolation of these values leads to an annual production of 5.9, 2.9 and 2.6 kg of raw screenings per inhabitant per year. The higher specific amount for WWTP A is related to the relatively short sewer system the plant is connected to. Only a little disintegration of solids takes place during transport through the sewer system.

Figure 1 shows, for each hour of the day, the incoming mass of raw screenings related to the total amount of raw screenings in the test period. As expected, the daily distribution of incoming screenings showed significant fluctuations. A first analysis shows two periods with high amounts of screenings appearing during 6 am to 1 pm and during 4 pm to 8 pm. The intensity and the time of the peaks differ from plant to plant. A low amount of incoming screenings was recorded during the late night and early morning hours. A strong increase followed in the morning. Depending on the length of the sewer system, the arrival time and characteristics of the peaks changed. Compared to the sewer systems of WWTPs B and C, the length of sewer system connected to WWTP A is short, which results in a distinctive peak between 9 and 10 am.

Energy content of raw screenings and single components

The COD concentration of raw screenings dry matter did not vary much over the day and showed similar results for every investigated WWTP, cf. Figure 2, with minor exceptions: between 1 am and 5 am the COD values for raw screenings were higher for WWTP A than the daily average. This might be caused by lower elution of parts of the screenings due to a lower water flow during the night compared to the water flow during the day. However this effect could not be observed for the screenings arriving at the other plants. Another explanation may be the small amount of incoming raw screenings during the night (see Figure 1).

Figure 1

Daily relative distribution of screenings from three different WWTPs.

Figure 1

Daily relative distribution of screenings from three different WWTPs.

Figure 2

Hourly variations of chemical energy of raw screenings expressed as COD of dry matter for three WWTPs.

Figure 2

Hourly variations of chemical energy of raw screenings expressed as COD of dry matter for three WWTPs.

Mass weighted average values for the incoming screenings were found to be 1.357 gCOD/gdm (A), 1.326 gCOD/gdm (B) and 1.362 gCOD/gdm (C). This shows that the chemical energy of mixed raw screenings is quite similar across the three investigated plants. To find out how much each component of raw screenings accounts for in terms of COD, the two main components (faeces F and toilet paper T) were investigated. Based on visual examination, other components were not considered in detail because of their low proportion of the heterogeneous mixture of substances and because of their characteristics (plastics are not elutable by washing). Four single (S) grab samples of each isolated component were analysed. Additionally, a mixed sample (M1F) of collected faeces was analysed. The results are shown in Table 3.

Table 3

Chemical energy content of toilet paper and faeces [gCOD/gdm]

Sample Toilet paper Faeces 
S1T 1.470 – 
S2T 1.130 – 
S3T 1.310 – 
S4T 1.250 – 
S1F – 1.740 
S2F – 1.620 
S3F – 1.720 
S4F – 1.710 
M1F – 1.660 
Sample Toilet paper Faeces 
S1T 1.470 – 
S2T 1.130 – 
S3T 1.310 – 
S4T 1.250 – 
S1F – 1.740 
S2F – 1.620 
S3F – 1.720 
S4F – 1.710 
M1F – 1.660 

The measurements show a higher energy content of faeces compared to toilet paper. Faecal energy content depends on nutrition behaviour. Faeces from vegans or vegetarians should result in higher energy content because of a higher proportion of undigested plant fibres (Koppe & Stozek 1999). The nutrition behaviour associated with the faeces investigated is unknown because the samples were taken directly from the screen. Faeces leading to the mixed sample M1F were collected from WWTP E during a 2 hour period. In total 5 kg of faeces were homogenised (dry matter content: 27.7%, loss on ignition: 89.6%).

The appearance of single pieces of faeces showed that elution took place in the sewer system. This leads to losses of chemical energy from the solid to the liquid phase of the wastewater.

Washing of raw screenings

Knowing the amount and the chemical energy content of raw screenings does not answer the question of how much energy can be recovered from screenings. By washing screenings in a screenings wash press, elutable components can be transferred from the solid to the liquid phase and subsequently converted to biogas in an anaerobic digester. The photographs in Figures 3 and 4 show a wash tank filled with 477 litres of water and 53 kg of raw screenings before and after washing them. The impeller (not visible) operated for 5 minutes, before the wash water left the washing chamber through a 5 mm sieve (not visible).

Figure 3

Mixture of raw screenings and wash water before washing.

Figure 3

Mixture of raw screenings and wash water before washing.

Figure 4

Mixture of eluted screenings and loaded wash water after washing.

Figure 4

Mixture of eluted screenings and loaded wash water after washing.

The potential of recovering easily biodegradable matter by washing the screenings was obtained by relating the COD load in the wash water to the dry mass of the raw screenings (see Table 4).

Table 4

Recovery ratio η, defined as the COD load of wash water per dry mass of raw screenings (raw screenings from six different municipal WWTPs)

WWTP 
Mean recovery ratio η [gCOD/gdm0.27 0.31 0.32 0.60 0.62 0.39 
Standard deviation ±0.03 ±0.03 ±0.03 ±0.03 – – 
Number of batches of raw screenings washed at each WWTP and considered in η 
WWTP 
Mean recovery ratio η [gCOD/gdm0.27 0.31 0.32 0.60 0.62 0.39 
Standard deviation ±0.03 ±0.03 ±0.03 ±0.03 – – 
Number of batches of raw screenings washed at each WWTP and considered in η 

The average ratios η shown in Table 4 range from 0.27 to 0.62 grams of recovered COD per gram of dry matter in the raw screenings. The upper value is derived from an experiment based on only one batch of raw screenings but is however validated by two batches analysed from WWTP D. Elution of raw screenings depends on their composition. Whereas faeces were found to be easily and completely elutable, not all particulate components were transferred to the wash water. The high η-values in WWTPs D and E indicate a high fraction of faeces in their raw screenings. It has to be taken into account that screenings were collected over 24 hours only for WWTPs A, B and C, whereas grab samples were taken from the other plants. Therefore, only the ratios for WWTPs A, B and C can be considered as daily averages. The raw screenings from WWTPs D, E and F were gathered in the morning hours and potentially contained more faeces than at other times of the day. This would explain the higher η-values reported in Table 4 for these plants, as faeces were found to be completely elutable.

Results related to the efficiency of the screenings wash process are shown in Table 5. The number of batches was the same as the numbers given in Table 4.

Table 5

Efficiency of transferring chemical energy of raw screenings into wash water

WWTP 
Efficiency of screenings wash press [%] and standard deviation 19.6 23.3 23.2 41.6 47.0 28.0 
±2.1 ±2.2 ±2.0 ±6.1 – – 
WWTP 
Efficiency of screenings wash press [%] and standard deviation 19.6 23.3 23.2 41.6 47.0 28.0 
±2.1 ±2.2 ±2.0 ±6.1 – – 

The calculation of the efficiency shows a wide range. The main reason for this finding is the different composition of screenings. Faeces are easily elutable, whereas toilet paper is not so the high values in Table 5 indicate a high proportion of faeces.

Energy recovery from screenings by washing

To assess the energy recovery potential of the process, the COD load recovered by washing the screenings was compared to the COD load of the plant influent for WWTPs A, B, and C. To calculate the COD load of the influent an average specific load of 120 gCOD/(PE*d) was assumed. For WWTP A 0.54% of the influent COD load was recovered by washing the screenings, with 0.29% and 0.30% for WWTPs B and C, respectively. These relative values show a minor importance of washing screenings for COD recovery. Investigated screenings were collected from screens with a bar spacing/slot width of 6 mm. The sewer systems for WWTPs B and C are quite long, which leads to a partial elution of faeces during transport through the sewer. Branner (2013) reported a higher yield of screenings for reduced bar spacing/slots of the screens. Subsequently a higher COD recovery can be expected. Additionally, it has to be underlined that only particulate matter can be recovered by screens. The total COD load in the WWTP influent used for comparison in Table 4 includes particulate as well as dissolved COD, and the latter cannot be removed by screens. Furthermore, it has to be mentioned that the perforated metal sheet used for separation of wash water and treated screenings contained holes with a diameter of 5 mm. A larger diameter would lead to a higher COD concentration of the wash water due to a shifted separation cut. This could cause operational problems during pumping of the wash water because of larger particle sizes.

The energy required to drive the screenings wash press was between 0.004 and 0.04 kWhel/kgdm, depending on the operational settings.

Based on results from Table 4 and the assumptions from the ‘Material and methods’ section, 0.18 kWhel/kgdm can be recovered. Hence, the conversion of recovered COD into electrical energy clearly exceeds the energy input by 4.6 to 46 times.

Washed screenings

Alongside the recovery of chemical energy, another benefit of washing out carbon-rich substances from the raw screenings is a reduction of their weight and thereby their disposal costs. As mentioned before, a fraction of the raw screenings is transferred into the wash water, thus reducing the mass of screenings for disposal. This needs additional water to operate the screening wash press but this can be taken from the effluent of the WWTP so that no fresh water is needed. Additionally, the results of this study showed that washed screenings have better dewatering properties than untreated screenings. The dry matter content of compacted washed screenings was 44% whereas compacted screenings without washing reached 38%. An explanation for this observation can be the difference in the screenings composition. Faeces can act as lubricants during the dewatering phase, increasing the pressure needed for a more effective dewatering of the screenings.

CONCLUSIONS

Availability of data concerning the amount of raw screenings arriving at WWTPs was improved by recording screenings hourly from three WWTPs within a period of 24 hours. Different diurnal cycles were observed. Common to all investigated WWTPs were the very low amounts of screenings during the late night and early morning hours. Depending on the sewer system, different peak characteristics were observed. Specific amounts of raw screenings per inhabitant per year vary significantly depending on the length of the sewer systems.

The energy content of mixed, homogenised raw screenings was found to be very similar for each investigated WWTP. Isolated components of screenings were analysed as well. Faeces showed a higher specific energy content than toilet paper. In terms of energy recovery, washing of screenings seems to be a process with minor relevance. Recovered COD by washing screenings was found to range from 0.27 to 0.62 gCOD/gdm. Screens with bar spacing/slot width sizes smaller than those investigated may show better results in terms of energy recovery. Washing of screenings with a higher proportion of faeces might also deliver better recovery ratios.

From an economic point of view the mass reduction of the treated screenings is an important factor in terms of disposal costs.

The COD load transferred into the washing water can ideally be used for anaerobic digestion in order to improve the energy efficiency of a WWTP. The fraction of biodegradable and non-biodegradable COD should be analysed in future studies. When sent directly to digesters, washing water does not need to be treated in the conventional biological step, which leads to energy savings due to a lower oxygen demand in the aeration tanks.

ACKNOWLEDGEMENT

The authors would like to express their sincerest thanks to the Federal Ministry of Education and Research for funding the project E-Klär (Reference 02WER1319; www.e-klaer.de). Thanks to the water board Ruhrverband for supporting the investigations, to Mrs Tasja Koyro and Mr Henrik Simianer for their support in experimental trials and to Huber, SE (Berching, Germany) for constructing the pilot plant.

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