The effect on the environment of the establishment and operation of a sludge treatment reed bed system (STRB) is quite limited compared to mechanical sludge dewatering, with its accompanying use of energy and chemicals. The assessment presented here of the investment, operation and maintenance costs of a typical STRB, and of the related environmental impact, is based on the experiences gained from the operation of a large number of STRB in Denmark. There are differences in the environmental perspectives and costs involved in mechanical sludge dewatering and disposal on agricultural land compared to STRB. The two treatment methods were considered for comparison based on a treatment capacity of 550 tons of dry solids per year and with land application of the biosolids in Denmark. The initial capital cost for STRB is higher than a conventional mechanical system; however, an STRB would provide significant power and operating-cost savings, with a significant saving in the overall cost of the plant over 20–30 years. The assessment focuses on the use of chemicals, energy and greenhouse gas emissions and includes emptying, sludge residue quality and recycling. STRB with direct land application is the most cost-effective scenario and has the lowest environmental impact. A sludge strategy consisting of an STRB will be approximately DKK 536,894–647,636 cheaper per year than the option consisting of a new screw press or decanter.

INTRODUCTION

Sewage sludge is a byproduct of wastewater treatment in municipal wastewater treatment plants (WWTP). In recent decades, there has been a growing focus on the treatment and handling of the vast amount of sludge produced and on the negative effects of the disposal of sludge in the environment. In Denmark, municipal WWTP produce over 1.3 million tons of sludge every year. The total dry solid (DS) content in the sludge produced from municipal WWTP has, in the period from 1995 to 2009, ranged from 132,000–162,000 tons of DS (latest available data, Kirkeby et al. 2013). The primary final disposal route for dewatered sludge in Denmark is to agricultural land. For instance, in 2009, 144,000 tons of DS was produced; of this, more than half (75,000 tons DS; 52%) was spread on agricultural land, 34,000 tons DS (24%) was incinerated and 17,000 tons DS (12%) was applied to sludge treatment reed beds systems (STRB) before being spread on agricultural land. The rest (16,000 tons DS; 12%) was either exported or put to landfill (Kirkeby et al. 2013). There are several technologies available on the market for sludge treatment and dewatering, ranging from traditional mechanical treatment by decanter and screw-press technology, to the environmentally friendly, but more land-area-demanding STRB systems. STRB are very cost-effective systems compared to traditional mechanical solutions (Uggetti et al. 2011, 2012). Compared to mechanical treatment, the dewatering of sludge in STRB occurs without the use of chemicals and with minimum energy consumption.

Legislation for sludge application

During the 1990s and onwards, stricter legislation has been brought into effect by the Danish Environmental Protection Agency to regulate the content of nutrients, heavy metals and hazardous organic compounds in sludge being spread on agricultural land (Nielsen 2005). In order to apply sludge on farmlands in Denmark, the following criteria for sludge quality (Table 1) has to be fulfilled. For certain heavy metals (cadmium (Cd), nickel (Ni), lead (Pb), mercury (Hg)), legal limit values for the application to farmland are related to both DS content (mg per kg DS) and to the total phosphorous (mg per kg TP) content in the sludge (Table 1). Contrary to current EU legislation (86/278/EEC EU Directive), Danish legislation (BEK No. 1650 of 13 December 2006) also sets upper limits for hazardous organic compounds: linear alkyl benzene sulfonates (LAS), nonylphenol/nonylphenol ethoxylates (NPE), di-2ethylhexyl-phtalate (DEHP) and polyaromatic hydrocarbons (PAH). Before application to agricultural land in Denmark, the sludge residue has to be analyzed for hazardous organic compounds. Currently, the EU Commission (Environment) is working on a proposal including limits for hazardous organic compounds for a new EU directive with stricter limits (ENV.E3 (2000) Working document on sludge, 3rd draft), but it has not yet been adopted. STRB is mentioned as a sludge treatment method in the proposal.

Table 1

Danish and European Union (EU) legal limits for heavy metals and hazardous organic compounds in sludge residue for agricultural use

  Denmark
 
EU
 
Limit values BEK No. 1650 of 13 December 2006  86/278/EEC EU Directive  ENV.E3 (2000) Working document on sludge, 3rd draft  
Heavy metals mg/kg DS mg/kg TP mg/kg DS mg/kg DS 
 Cadmium (Cd) 0.8 100 20–40 10 
 Copper (Cu) 1000 – 1000–1750 1000 
 Nickel (Ni) 30 2500 300–400 300 
 Lead (Pb) 120 10000 750–1500 750 
 Zinc (Zi) 4000 – 2500–4000 2500 
 Mercury (Hg) 0.8 200 16–25 10 
 Chromium (Cr) 100 – – 1000 
Organic contaminants mg/kg DS mg/kg TP mg/kg DS mg/kg DS 
 LAS 1300 – – 2600 
 PAH – – 
 NPE 10 – – 50 
 DEHP 50 – – 100 
  Denmark
 
EU
 
Limit values BEK No. 1650 of 13 December 2006  86/278/EEC EU Directive  ENV.E3 (2000) Working document on sludge, 3rd draft  
Heavy metals mg/kg DS mg/kg TP mg/kg DS mg/kg DS 
 Cadmium (Cd) 0.8 100 20–40 10 
 Copper (Cu) 1000 – 1000–1750 1000 
 Nickel (Ni) 30 2500 300–400 300 
 Lead (Pb) 120 10000 750–1500 750 
 Zinc (Zi) 4000 – 2500–4000 2500 
 Mercury (Hg) 0.8 200 16–25 10 
 Chromium (Cr) 100 – – 1000 
Organic contaminants mg/kg DS mg/kg TP mg/kg DS mg/kg DS 
 LAS 1300 – – 2600 
 PAH – – 
 NPE 10 – – 50 
 DEHP 50 – – 100 

STRB systems are a widespread and common sludge treatment practice in northern Europe, particularly in Denmark, where the technology has been used for more than 26 years (Nielsen et al. 2014). The treatment period in STRB is approximately 10–12 years, but up to 20 years has been achieved (Nielsen et al. 2014). Several STRB systems in Denmark have been in operation for more than 15–20 years and have been through one emptying period; some have been through a second emptying period.

In the present study, two treatment methods were considered for comparison in the economic and environmental assessment, including biomass production, benefits and ancillary benefits. In general, sludge production consists of surplus activated sludge (SAS) directly from the activated sludge plant (0.4–0.6% DS) or SAS from final settling tanks (1.0–1.5% DS).

MATERIAL AND METHODS

The present study shows data following 10–20 years of treatment in STRB systems situated in Denmark. Both options are calculated on the basis of a treatment capacity of 550 tons of DS per year. Construction and operational costs are estimated for the two options:

  • mechanical dewatering in a new screw press or a new decanter with land application of stored sludge;

  • sludge treatment in a new STRB with 10 basins and an area load of 60 kg DS/m2 year with direct land application of the biosolids.

Sludge treatment reed bed systems

The overall sludge reduction takes place in the reed-planted basins in two ways: partly due to dewatering (draining and evapotranspiration) and partly due to mineralization of the organic solids in the sludge. The overall reduction of the sludge volume occurs without the use of chemicals such as polymers. The process involves a very low level of energy consumption for pumping the sludge and reject water (Nielsen 2003). After the final treatment, the sludge residue will typically end up having a DS content of 20% and higher, reaching 35% depending on the sludge quality treated in the systems (Nielsen 2011). The STRB is emptied gradually, with approximately two of the basins selected for emptying per year. Maintaining full capacity during the emptying phase is possible if the basins are re-established after emptying, either by sufficient regeneration of vegetation or replanting. The quality of the sludge residue in the STRB meets the criteria set out in the statutory order on sewage sludge (BEK No. 1650 of 13 December 2006) (Table 1) from the Ministry of the Environment (Denmark), and the final disposal of residual sludge will be to agricultural land.

Mechanical dewatering

SAS from the final settling tanks is further concentrated in a concentration tank (volume 30 m3) to approximately 2–4% DS. On a daily basis, the sludge is pumped to a buffer tank (volume 60–70 m3), including aeration and stirring before pre-dewatering. The pretreatment includes the use of polymers and pumping facilities, approximately 2,115 kg of DS of sludge is pressed 7 hours a day for 260 days per year, which gives the necessary capacity of 300 kg DS/hour. From pretreatment, the sludge is finally dewatered including the use of polymers and pumping facilities by mechanical screw press or decanter. Danish legislation requires storing the dewatered sludge for 9 months before recycling the sludge to agriculture after harvesting. The dewatered sludge is transported in containers to the sludge stockpiling areas. As part of the WWTP, an area for stockpile can be established to provide storage for dewatered sludge before final disposal to agriculture.

The capital cost (investment cost) estimates for the STRB and mechanical dewatering are based on the above sizing. The yearly depreciation is estimated at a 3–5% interest rate over 15–30 years for the construction costs and is based on Equation (1): A: price, a: depreciation, r: interest and n: number of years 
formula
1

RESULTS

STRB technology is a low-energy, low-operational-cost process, which provides long-term sludge treatment. In order to compare the two treatment methods, sizing, capital and operating cost estimates and power use estimates were developed for an STRB. Likewise, estimates for conventional mechanical dewatering systems were developed for comparison purposes. The comparison includes flexibility and the working environment in sludge treatment systems.

Construction and operational costs for STRB

The following budget estimate is based on an STRB with 10 basins, fully automated controls and DS and flowmeter instrumentation, Scada system including data processing, and with an optimal filter for effluent runoff.

Construction cost

Capital cost (investment cost) estimates for the STRB, based on the above sizing, process description and plantation of four reed plants/m2, has been estimated at DKK 11,500,000. Estimation of the yearly depreciation (Figure 1) at 3% interest rate over 30 years for construction costs amounts to DKK 586,500 (Table 5).

Figure 1

A comparison of the estimated yearly operational cost (y axis) including depreciation of the investment for mechanical dewatering equipment versus STRB (both with disposal on agricultural land) for a population of 50,000 PE in England over a 15-year period.

Figure 1

A comparison of the estimated yearly operational cost (y axis) including depreciation of the investment for mechanical dewatering equipment versus STRB (both with disposal on agricultural land) for a population of 50,000 PE in England over a 15-year period.

Operational costs

The operational cost for the STRB is based on treatment of 550 tons DS/year, on an expected final DS of 25–30% and a mineralization of 25%, equivalent to 12,400–16,500 tons of sludge at the time of emptying after approximately a 10-year life cycle.

With the price for agricultural application at DKK 250 per ton (taken from the current sludge distributor), the cost of emptying will be DKK 3,100,000–4,125,000. To cover this cost, an annuity of DKK 363,320–483,450 per year should be set aside (at an interest rate of 3% over 10 years (Equation (1)). The yearly operational cost (excluding VAT) is (Table 2) estimated at DKK 458,120–578,250 (excluding depreciation of construction costs). The total cost estimate (including system depreciation) of the total yearly costs for STRB, according to the aforementioned prerequisites and conditions amounts to approximately DKK 1,045,620–1,164,750 for sludge handling (Table 5), equivalent to approximately DKK 1,909–2,116 per ton of DS for sludge treatment via STRB and final disposal to agricultural land.

Table 2

Estimation of operational costs

Operation activity Costs (DKK)f 
Maintenancea 30,000 
Power consumptionb 6,000 
Polymer 
Staff-hoursc 46,800 
Analysisd 12,000 
Emptying, transportation, final application on lande 363,320–483,450 
Total 458,120–578,250 
Operation activity Costs (DKK)f 
Maintenancea 30,000 
Power consumptionb 6,000 
Polymer 
Staff-hoursc 46,800 
Analysisd 12,000 
Emptying, transportation, final application on lande 363,320–483,450 
Total 458,120–578,250 

aCosts for the maintenance and calibration of mechanical equipment (pumps, valves, density and flow meters).

bPower consumption primarily comprised of power to the pumps for sludge pumping and return pumping of effluent. The pumps, which are approximately 3 kW, have approximately 1,500 operational hours per year. The automatic valves, density and flow meters, and lighting also use power. Total expected consumption is approximately 6–8,000 kWh/year.

cStaff-hour budget for the landscaping maintenance and operations is calculated on the basis of experience from similar systems to be approximately 3 hours per week at DKK 300 per hour.

dGenerally, four samples are taken per year with regard to operations (loss-on-ignition, pH and DS, heavy metals and fat/oil). Each analysis costs approximately DKK 3,000.

eSTRB is expected to be emptied after a 10-year cycle (typically over a span of 4–5 years for 10 basins).

fExchange rate: 100 Euro = 765 DKK.

Construction and operational costs for mechanical sludge dewatering

Capital cost estimates (investment cost) for the decanter and screw-press are based on the above sizing and the process description.

Construction cost

For investment in mechanical dewatering equipment, construction costs for its establishment are based on investment in buildings, buffer tanks, polymer stations, etc. (Table 3).

Table 3

Cost estimates for screw press or centrifuge dewatering system

Investments Budget (DKK)a 
Sludge dewatering system 1,300,000–1,600,000 
Dewatering building (area: 35–40 m2400,000–500,000 
Concentration tank (volume: 30 m3125,000 
Buffer tank (volume: 60–70 m3180,000 
Pretreatment 250,000 
Stockpile area (capacity: 9 months production) 1,800,000 
Change to existing system 100,000 
Unforeseen expenses (10%) 415,500–455,500 
Total 4,570,500–5,010,500 
Investments Budget (DKK)a 
Sludge dewatering system 1,300,000–1,600,000 
Dewatering building (area: 35–40 m2400,000–500,000 
Concentration tank (volume: 30 m3125,000 
Buffer tank (volume: 60–70 m3180,000 
Pretreatment 250,000 
Stockpile area (capacity: 9 months production) 1,800,000 
Change to existing system 100,000 
Unforeseen expenses (10%) 415,500–455,500 
Total 4,570,500–5,010,500 

aExchange rate: 100 Euro = 765 DKK.

Operational costs

Electricity and chemicals are included in the calculation of operational costs. The price of electricity is set at its current price. Internal (WWTP) costs for the transportation and disposal of sludge are not included. The unit prices in Table 4 are used in the calculation of operational costs.

Table 4

Operational costs for screw press or centrifuge dewatering system

Operation activities Unit price Yearly cost (DKK)a 
Premixed polymer (50% active) 25 DKK/kg  
Yearly polymer cost: 550 tons DS*10 kg/tons DS*2  275,000 
Pretreatment polymer cost: 550 tons DS*3 kg/tons DS*2  82,500 
Electricity 0.75 DKK/kwh  
Total yearly electricity cost (5h*260d'5 kWh)  4,875 
Hourly labor rate for operational staff 300 DKK/hour  
Labor, equipment operations (2 hours/day*260 days)  156,000 
Maintenance  50,000–100,000 
Analysis (four samples) 3,000 12,000 
Emptying, transportation and final application on land 250 DKK/m3  
Emptying, transportation, application on land (2,000–2,750 m3 550,000–687,000 
Total cost  1,130,375–1,317,375 
Operation activities Unit price Yearly cost (DKK)a 
Premixed polymer (50% active) 25 DKK/kg  
Yearly polymer cost: 550 tons DS*10 kg/tons DS*2  275,000 
Pretreatment polymer cost: 550 tons DS*3 kg/tons DS*2  82,500 
Electricity 0.75 DKK/kwh  
Total yearly electricity cost (5h*260d'5 kWh)  4,875 
Hourly labor rate for operational staff 300 DKK/hour  
Labor, equipment operations (2 hours/day*260 days)  156,000 
Maintenance  50,000–100,000 
Analysis (four samples) 3,000 12,000 
Emptying, transportation and final application on land 250 DKK/m3  
Emptying, transportation, application on land (2,000–2,750 m3 550,000–687,000 
Total cost  1,130,375–1,317,375 

aExchange rate: 100 Euro = 765 DKK.

It is expected that DS content in the dewatered sludge is approximately 20–25%. The yearly sludge handling costs (including system depreciation) amount to approximately DKK 1,582,514–1,812,386 or 2,877–3,295 per ton DS (Table 5) for dewatering by mechanical dewatering equipment and disposal on agricultural land (price based on an offer from the current sludge distributor).

Table 5

A comparison of the estimated yearly operational cost for mechanical dewatering equipment versus STRB and both with deposition on agricultural land

DKKa Mechanical dewatering STRB 
Depreciation of the investment cost 440,139–482,511b 586,500d 
Analysis 12,000c 12,000 
Operational costs 580,375–630,375 82,800 
Final deposition 550,000–687,500 363,320–483,450 
Total per year 1,582,514–1,812,386 1,045,620–1,164,750 
DKKa Mechanical dewatering STRB 
Depreciation of the investment cost 440,139–482,511b 586,500d 
Analysis 12,000c 12,000 
Operational costs 580,375–630,375 82,800 
Final deposition 550,000–687,500 363,320–483,450 
Total per year 1,582,514–1,812,386 1,045,620–1,164,750 

aExchange rate: 100 Euro = 765 DKK.

bConstruction costs for mechanical dewatering equipment with depreciation over 15 years at 5% interest.

cMetering of DS for the optimization of the mechanical dewatering equipment as well as documentation for heavy metals and hazardous organic compounds.

dConstruction costs for STRB with depreciation over 30 years at 3% interest.

Comparison of the two options

A sludge strategy consisting of an STRB will be approximately DKK 536,894–647,636 cheaper per year than the option consisting of a new screw press or decanter, respectively (Table 5). The estimation of construction cost depreciations is based on the formula in Equation (1).

DISCUSSION

There are important differences in the environmental perspectives and costs involved in mechanical sludge dewatering followed by disposal on agricultural land compared to STRB. The information in Table 5 illustrates that, while the capital cost of the STRB is higher than that of the mechanical options presented, the reverse is true of the system operating costs. This comparison shows clearly the significant lifetime cost savings.

The STRB in Denmark would provide significant yearly power and operating cost savings. In the overall cost of the plant over an operations period of 15–30 years, there are significant savings compared to mechanical treatment systems, not only for a system with a capacity of 550 tons of DS per year but based on experience also for both small and large STRB with a treatment capacity between 100 and 3,500 tons of DS yearly. A similar comparison, carried out by the author, in the economic assessment between a centrifuge and STRB for a population of 50,000 PE in England obtained a similar result, illustrating the effect that this difference in operational costs has over the design life of the plants (Figure 1). A similar comparison in Spain found that STRB with direct land application is the most cost-effective scenario, which is also characterized by the lowest environmental impact (Uggetti et al. 2011).

Environmental impact

An STRB produces a high-quality sludge that is suitable for application to agricultural land as a soil improver and fertilizer. The content of nutrition is high, so the sludge can be used as a substitute for commercial fertilizer (Kołecka & Obarska-Pempkowiak 2013; Nielsen & Bruun 2014). Under Danish climatic conditions, the STRB achieves a final DS content of 20–35% compared to DS content in mechanical dewatered sludge, which is approximately 20–25%. The environmental benefits of STRB technology can be summarized as follows: no chemical use (polymers) for dewatering, minimal power consumption and minimal emissions and environmental impact.

Hazardous organic compounds

Studies show that hazardous organic compounds are stabilized and mineralized during treatment in STRB (Federle & Itrich 1997; Matamoros et al. 2012; Peruzzi et al. 2011, 2013; Nielsen 2005; Nielsen et al. 2014). In a study of the mineralization of LAS and NPE in digested sludge treated in an STRB, a degradation of 98% of LAS and 93% of NPE was observed under aerobic conditions (Nielsen 2005). In the same study, reductions of approximately 60% and 32% were obtained for DEHP and PAH, respectively. The organic pollutants were mineralized – not only the upper layers of sludge but in the whole depth (Nielsen 2005) – to such a degree that the sludge conforms to the limits and norms for disposal on agricultural land (Table 1). In the storage experiments with anaerobically digested sludge (representing mechanical treatment with 9 months' storage of sludge before land application) LAS, NPE, DEHP and PAH were only partly degraded in the top layer (0–20 cm) and below 20 cm there was no degradation at all. Degradation under anaerobic conditions was negligible (Nielsen 2005; Aagot et al. 2000). The study demonstrated that oxygen was the limiting factor in the degradation of organic pollutants while temperature has a high level of influence on the rate of degradation. Oxygen influx into the sludge considerably improved the mineralization of LAS, NPE, DEHP and PAH, while mineralization under anaerobic conditions was very limited. Hazardous organic compounds are not stabilized and mineralized during mechanical dewatering. Thus, operation costs for mechanical dewatering could increase because the sludge does not meet the limits (Table 1) and consequently cannot be recycled to land. The sludge would need further treatment via composting or would need to be applied to a landfill with disposal costs ranging from 1,800–2,250 DKK/ton DS, or 4,000–5,000 DKK/ton DS, respectively. The sludge quality from an STRB is cleaner and more adaptable in the natural cycle than mechanically dewatered sludge (Nielsen 2005). Another problem for mechanical dewatering could arise as it is possible that from 2017 the use of polymers may not be allowed in Germany (Düngemittelverordnung Dezember 2012) in the mechanical dewatering process unless a minimum of 20% of the polymers are mineralized 2 years after the sludge has been applied to agricultural land.

Emissions

Investigations of methane gas production from the Helsinge STRB demonstrate that there was production of methane from the wet sludge residue in the first days after the loading phase until the residual layer became suitably dewatered (Nielsen 2011). In another study, where the aim was to measure CO2 and CH4 emissions from an STRB and an occasionally loaded sludge depot (SD), the STRB emitted twice as much CO2 (1200 mg/m2/hour) as the SD, whereas the SD emitted four times more CH4 (2 mg/m2/hour) than the STRB. It was concluded that STRB with more aerobic conditions in sludge residue result in low CH4 emission rates and have a low climate impact relative to conventional treatment alternatives, but that overloading and poor sludge management of an STRB could increase the emissions CH4 emissions (Olsson et al. 2014). The life cycle assessment highlights that global warming has a significant impact in all scenarios, which is attributed to fossil fuel and electricity consumption, while greenhouse gas emissions from STRB are insignificant (Uggetti et al. 2011).

Pathogenic microorganisms

Analysis of the reduction in pathogens in the sludge residue in an STRB during a period of 1–4 months after the last loading indicated that the pathogen content was reduced. The reduction of the pathogenic microorganisms means that the sludge residue from the STRB is cleaner and better suited for recycling on agricultural land than mechanically dewatered sludge (Nielsen 2007).

CONCLUSIONS

In this study, the differences in the environmental perspectives and costs involved in STRB sludge treatment and disposal on agricultural land were compared to mechanical sludge handling. In order to compare the two treatment methods sizing, capital and operating cost estimates, and power use estimates were developed for comparison purposes under Danish conditions. The final sludge residue after a 10–20 year treatment and stabilization period in an STRB complies with the limits in both Danish and EU legislation for sludge. The sludge residue has a quality that makes it suitable for land application. The content of nutrients is high, so the sludge can be used as a substitute for commercial fertilizer. Experience shows that operation of an STRB is flexible, with low operational cost due to low energy consumption, minimal operator supervision maintenance requirements, and no requirement for pretreatment (thickening, dewatering). A sludge strategy consisting of an STRB will be approximately DKK 536,894–647,636 cheaper per year than the option consisting of a new screw press or decanter.

The capital cost of the STRB is higher than that of the mechanical options presented; the reverse is true of the system operating costs. This comparison shows clearly that the STRB would provide low environmental impact and significant power and operating cost savings with a significant saving in the overall cost of the plant over an operation period compared to mechanical treatment systems.

ACKNOWLEDGEMENTS

The author wishes to thank the technical staff at the Helsinge WWTP and the Kallerup WWTP for their technical assistance during operations and sampling.

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