# Presentation: 403853

## End Use of Sewage Sludge in Cement ManufacturingPotential Economic and Environmental Benefits

- Ashley Murray, PhD Student
- Energy and Resources Group
- October 1, 2007

## What is Sludge?

- Byproduct of wastewater treatment
- Generated in primary, secondary and advanced wastewater treatment processes
- 70-90 g/capita/day;  1DT/10,000 person equivalent
- >1 M DT/yr in China
- 5% dry solids
- High nutrient concentration, high pathogen concentration, potentially high metal concentration

## Sludge End Use Options

- Disposal in landfill
- Land application (composted or digested)
- Building materials
- Cement, clay brick, lightweight aggregate

## Why Use Sludge in Cement?

**Benefits to WWTP**
- Save money
- Avoided infrastructure and operation costs
- Avoid finding landfill willing to accept
- Contribute to “circular economy” and environmental sustainability

**Benefits to Cement Plant**
- Save Money
- Energy recovery from sludge
- Reduce environmental impact over lifecycle of manufacturing
- CO2, SO2, NOx, fuel
- Contribute to “circular economy” and environmental sustainability

## Case Study: Chengdu, Sichuan Province

| WWTP | Wastewater Flow (m3/d) | Sludge Production (dry tons/d) |
| --- | --- | --- |
| 1 | 426,000 | 45 |
| 2 | 150,000 | 19 |
| 3 | 73,000 | 10.1 |
| 4 | 58,000 | 9 |

- Profile of 4 WWTPs in Chengdu’s
- urban core

- Sludge Production
- 2006: ~84 DT/d; 2020: ~350 DT/d
- Sludge Lower Heating Value
-       2500 Kcal/kg DS = 10.5 MJ/kg DS

## Analytical Approach: Sludge End Use 

- Key Assumptions:
- Sludge is an inevitable by-product of wastewater treatment; past life *not* accounted for in end-use analysis
- Chengdu-specific energy, transportation, sludge characteristics

- Analytical Approach: Sludge End Use

## Annual Environmental Costs and Benefits for Cement Plant

|  | 20% DS | 36% DS | 67% DS |
| --- | --- | --- | --- |
| SO2 (kg)
 | 1.7x105 | -7.7x103 | -3.5x105 |
| NOx (kg)
 | 1.9x104 | -9.7x102 | -4.0x104 |
| GWP (kg CO2)
 | 7.2x106 | -3.3x105 | -1.5x107 |
| Electricity (kWh)
 | -2.2x103 | -2.2x103 | -2.2x103 |
| Fuel (MJ) | 7.6x107
 | -3.5x106
(25 MWe power plant) | -1.57x108
(1100 MWe power plant) |

- Sludge Dry Solids Content

## Annual Economic Costs and Benefits for Cement Plant

|  | 20% DS | 36% DS | 67% DS |
| --- | --- | --- | --- |
| Value/wet ton
(RMB) | 45 | -2 | -92 |
| Annual
(RMB) | 81,400 | -1500 | -37,700 |

- Sludge Dry Solids Content

## Factors that Impact Environmental and Economic Outcomes

- Lower Heating Value of sludge
- Fraction dry solids
- Transportation
- distance, volume
- Fuel source for heat drying
- coal or natural gas

## Transportation Sensitivity Analysis: Environmental Attributes

- Impact of Sludge Dry Solids Content

- Impact of Sludge Transportation Distance (36% DS Sludge)

- http://yosemite.epa.gov

## Transportation Sensitivity Analysis: Environmental Attributes

- Transportation Sensitivity Analysis: Environmental Attributes

- Impact of Sludge Dry Solids Content

- Impact of Sludge Transportation Distance (36% DS Sludge)

## Transportation Sensitivity Analysis: Economic Attributes

- Impact of Sludge Dry Solids Content

- Impact of Sludge Transportation Distance (36% DS Sludge)

## Fuel Source Sensitivity Analysis: Environmental Attributes

- Impact of Fuel Type for Heat Drying (36% DS)

- http://www.chinaexpat.com/files/u1/coal.jpg

- http://www.politicaldogs.org/uploaded_images/Coal-759168.jpg

## Fuel Source Sensitivity Analysis: Economic Attributes

- Impact of Fuel Type for Heat Drying
- (36% DS)

## Summary & Future Work

- Cement Factories can capture environmental and economic benefits from sewage sludge
- BUT...
- Schemes require strategic planning to optimize environmental sustainability
- Need for creative financing mechanisms or incentives to make environmentally optimal schemes economically feasible