Centralized Treatment: Scaled Up Testing of Chemical Additives for Sludge Treatment


To avoid disposal of untreated faecal matter into the environment, treatment options that can sanitize (kill pathogens) and stabilize faecal matter (reduce vector attraction), need to be employed in the immediate phase of emergencies. Several chemical and biological processes have been tested in the field for their efficacy in an emergency set-up. Multiple studies carried out to establish efficacy of treatments in the context of an emergency have concluded that treatment of faecal sludge using urea, lime and lactic acid fermentation can sanitize faecal sludge safely in between 3 and 15 days if certain conditions are provided[1].  However, these studies did not investigate the efficiency of faecal sludge treatment at large scale. 

Manufacturers of additives claim that their products increase the rate of decomposition. Earlier research on the use of additives to stabilize faecal matter gave variable results; some state that biological additives work[2] while others claim that they do not[3]. The observed differences may be attributed to the difference in stabilizers, method of application and management issues (number of users, almost full pits, ownership of facility and whether the pit was or was not in use during the experimental period). 

In the first phase of the ESP, WASTE and its sanitation partners carried out research to determine whether commercially available additives can stabilize and sanitize faecal sludge. The initial finding in ESP Phase 1 indicated that chemical additives were efficient in terms of pathogen reduction and organic matter decomposition.

Work Carried Out in Phase 2

As the Phase 1 testing was done at lab and small scale, the present research aimed to determine the treatment effectiveness of lime, caustic soda and urea in 2000 litre storage tank. The additives tested were:

No Additive name Product description Category
1 Soda Lab grade Sodium Carbonate Chemical
2 Lime Alkaline or Lime stabilization is a simple process which reduces odour, vector attraction and pathogen levels in wastewater and wastewater treatment sludges (also known as biosolids)[4]. The process involves the application of an alkaline substance such as calcium hydroxide (Ca(OH)2) to increase the pH and create a highly alkaline environment which is hostile to biological activity[5]. Chemical
3 Urea Urea Treatment is based on the sanitizing effect of uncharged ammonia (NH3) which has been demonstrated to be a harmless chemical substance capable to efficiently inactivating bacteria[6]. Chemical

The test parameters, reagents, apparatus and analytical method used for the analysis were:

Parameter Reagents/ Materials/Glassware Apparatus Method
  Total COD   Digestion solution (Water, K2Cr2O7, conc. H2SO4, H2SO4/Ag2SO4) Stock solution (Potassium hydrogen phthalate dissolved in water)   Digestion vessels Oven to operate at 150±2°C Spectrophotometer to operate at 600 nm   Standard method SM 5220D Closed reflux method
  Total solids and Volatile solids     Oven, 105°C Analytical balance (10mg accuracy) Evaporating dish Muffle furnace   Standard method SM 5540G Gravimetric method
pH   WTW pH 340i field meter  
Temperature   Mercury Thermometer  
  E. coli   Chromocult coliform agar(Merck Millipore) Distilled water Cotton wool Aluminium foil Peptone Sodium chloride   Autoclave Incubator, (37±2°C) Water bath controlled thermostatically at, 100°C pH meter Burner flame Petri dishes (90mm) Glass spreader   ISO 9308-1 Surface plate method
  Enterococci   m Enterococcus agar (Difco) Distilled water Cotton wool Aluminium foil Peptone Sodium chloride   Autoclave Incubator, (35±2°C) pH meter Burner flame Petri dishes (90mm) Glass spreader   ISO 9308-1 Surface plate method

Two different experimental units were set-up. The first was done at WASTE Advisers office in Blantyre Malawi, where three storage tanks of 2000 litre capacity were loaded with faecal sludge collected from pit latrines and three types of additives were applied under two different conditions (mixed and non-mixed).  Mixing was conducted using a circulation pump. The second type of set-up was combined with the testing of the earth auger and Loowatt toilets and is described in that section of the report.


2000 litre storage tanks, filled up with faecal sludge


The characteristics of faecal sludge were tested at laboratory at WASTE office and at the Polytechnic University of Malawi in Blantyre. The characterization of faecal sludge used in the laboratory and field experiments respectively was:

Parameter Units  
Total COD mg O2/L 2,960 to 11,253
Total solids % 0.64 to 12.20
pH   7.27 to 7.9
Temperature ºC 18.5 to 25.1
E. Coli CFU/100mL 1.93E+04 to 4.25E+05

The concentrations of E. Coli in treated black water was enumerated and compared to the WHO guideline value for restricted agriculture which is <1000 cfu faecal coliform/100ml (100g) of sample. When the additives were applied without mixing the concentrations of E. Coli increased, consequently no sanitization process happened.

When the additives were applied mixing the faecal sludge with them, a sanitization process took place. After one day of lime and urea application the faecal sludge was sanitized as no pathogens could have been identified. For caustic soda, three days after treatment and with daily mixing the faecal sludge was sanitized (concentrations below 1000 cfu faecal coliform/100ml (100g) of sample)

The parameter evaluated on this research to establish the stability of treated sludge was VS/TS ratio. A stable sludge has VS/TS less than 60% and has achieved a volatile solids reduction ≥ 38%. VS/TS of stable sludge was obtained using the equation below:

On the trial done without mixing the additives with faecal sludge, there was not significant reduction on the concentration of organic matter.

On the trial with a mixing condition, only the tank mixed with urea reached the volatile solids reduction level required to determine as stable sludge. Regarding the tanks mixed with lime and caustic soda, although there was a significant reduction of volatile solids, they could not be identified as stable sludge.

Based on this trial, we can conclude that, when using chemical additives such as lime, soda and urea, rapid stabilization and sanitization of faecal sludge is most likely to be achieved when they are mixed. Because of the type of sludge used on the test the mixing process, which used a membrane pump, was difficult and required multiple mixing sessions. Significant reduction in concentrations of E. coli was observed in faecal sludge treated with additives of chemical origin.  Lime also reduced the pathogen concentration faster than caustic soda. While lime and urea brought pathogen levels down to acceptable limits in one day, caustic soda demanded three days to reach the recommended level.

[1] de Pooter, 2014; González Pérez, 2014; Malambo, 2014; Nobela, 2014

[2] Jere et al., 1998; Taljaard et al., 2003

[3] Bakare, 2011; Buckley et al., 2008; Foxon et al., 2008

[4] Williford, Chen, Shammas, &Wang, 2007

[5] Schwing Bioset, 2009

[6] Vinneras, Nordin, Niwagaba, &Nyberg, 2008