2. LITERATURE REVIEW
2.5. Communal wastewater characteristics in developing countries
2.5.1. General
The main design parameters for communal DEWATS are the estimated per capita wastewater production, the peak flow factor and the per capita organic load. These parameters are strongly dependent on water availability, climate, culture and income. Water scarcity for example would logically lead to lower wastewater production and higher concentrations (FAO, 1992). Literature on per capita wastewater and organic waste production is however mainly available on western countries.
Engineers in developing countries are therefore forced to use such design values in the absence of more suitable estimates which may lead to oversized systems and resource wastage (Campos and vonSperling, 1996).
This section reviews the existing literature on wastewater characteristics in developing countries.
2.5.2. Feed flow characteristics
2.5.2.1. Per capita wastewater production
Wastewater production is influenced by numerous factors and intrinsically varies from one climatic zone to another, from country to country, from rural to urban areas and from city to city (UNEP, 2014).
The “International Benchmarking Network for Water and Sanitation Utilities” (IBNET) from the World Bank’s “Water and Sanitation Program” (WSP) provides a publically accessible database for international water and sanitation utilities performance data (WSP, 2014). Table 9 summarizes the most recent residential water consumption values for a number of DEWATS implementation relevant
countries. Communal wastewater production values are estimated as being 80% of the residential water consumption. No detailed information was available as for which population sections these values are representative. It can however be assumed that the data has a strong bias towards urban, high-income communities. The majority of the poor urban and rural populations in these countries relies primarily on water from private shallow wells and would therefore not be reflected in these numbers. The values given by the data for Kenya and Tanzania are remarkably low. The reason for this is not further investigated.
Table 9: Communal ww production in selected countries based on residential water consumption data as given in the IBNET/WSP database (WSP, 2014), all values in “l cap-1 d-1”
Continen
t Country Residential water consumption
Estimated communal
ww production* Year of inquiry
Africa Kenya 36 29 2010
Africa South
Africa 190 152 2009
Africa Tanzania 29 23 2009
Asia India 83 66 2009
Asia Indonesia 117 94 2004
Asia Cambodia 101 81 2007
Asia Vietnam 115 92 2011
Asia Philippines 117 94 2009
*estimated as being 80% of the residential water consumption; ww = wastewater
Campos and vonSperling (1996) analysed wastewater data from low-income communities in Brazil.
The authors found that the average household income correlated with the per capita wastewater production. They concluded that the generally adopted text book values based on data from western countries overestimate this value for low to middle income areas in Brazil which was found to be 50 l cap-1 d-1 to 100 l cap-1 d-1.
The WHO (WHO/UNEP, 1997) proposes different communal wastewater production ranges for industrial, developing and arid regions (see Table 10). Table 10 also contains further data from other authors on various African and Asian countries. However also here, most of the data stems from water and sanitation utility companies and certainly represents the urban rich more than the poor or rural population.
Crous (2013) measured an average water consumption of 47 l cap-1 d-1 at community ablution centres in South African informal settlements.
Table 10: Per capita communal ww production data from various sources Continent/
Region Country ww prod.
per cap* Details Comments Reference
General 85-200 Industrial regions WHO/UNEP (1997)
General 65-125 Developing regions WHO/UNEP (1997)
General 35-75 (Semi-) arid regions WHO/UNEP (1997)
Africa Yemen 80 City of Sana'a WHO/UNEP (1997)
West Asia 100 UNEP (2014)
West Asia Jordan 90 City of Amman FAO (1992)
East Asia Developing
countries 160-200 Water supply demand UNEP (2014)
East Asia Indonesia 160 Feed to septic tanks UNEP (2014)
East Asia Vietnam 150 Values used to calculate sewage by
municipalities UNEP (2014)
East Asia Vietnam 125 Cities > 3*106 pop. Estimated values, not measured UNEP (2014) East Asia Vietnam 69 Cities 1 - 3*106 pop. Estimated values, not measured UNEP (2014) East Asia Vietnam 39 Cities <106 pop. Estimated values, not measured UNEP (2014)
South Pacific Fiji 270 UNEP (2014)
East Asia Thailand 74 Bangkok Rural areas ^ Tsuzuki (2010)
East Asia India 143 Cities > 105 pop. # CPCB (2009)
East Asia India 97 Cities 5*104 - 105
pop. # CPCB (2009)
*in l cap-1 d-1; ^ Estimated through water usage data for toilet, bathroom, laundry and kitchen; # Estimated as 80% of the per capita water supply; ww = wastewater
2.5.2.2. Flow fluctuations
Communal wastewater flow characteristically fluctuates within seasonal, weekly and diurnal periods.
These fluctuations depend on numerous factors and certainly vary from site to site depending on climatic characteristics and water usage habits. Figure 13 presents an example of a typical diurnal wastewater flow pattern with low flow at night and during the afternoon and flow peaks in the morning and evening. The relative amplitude of these fluctuations can be regarded as being stronger the smaller the community is, since varying water usage habits across households are less evened out.
Figure 13: Diurnal variation of domestic water consumption (Haestad et al., 2004)
2.5.3. Typical concentrations
Campos and vonSperling (1996) analysed wastewater data from low-income communities in Brazil.
They concluded that the generally adopted text book values underestimate the wastewater concentration of low-income communities which was generally above 300 mg BOD5 l-1.
The UNEP (UNEP, 2014) confirms that local wastewater characteristics strongly depend on local conditions and habits such as nutrition level, staple food composition and kitchen habits. They therefore “vary from country to country, from rural to urban areas and from city to city” (UNEP, 2014) as well as from dry to wet climate. The ranges for general wastewater concentration values for developing and emerging countries reported by WHO/UNEP (1997) are therefore very large (see Table 11). Water scarce areas like Jordan for example feature very high concentrated wastewater.
Communal wastewater concentrations can therefore not be generalized and need to be assessed from case to case.
Table 11: Communal wastewater concentration characteristics in developing and emerging countries Continent/
Region Country Parameters (in mg l-1) Comment Reference
COD BOD5 NH4-N PO4-P
General
280-2500
120-1000 30-200* 4 to 50 WHO/UNEP
(1997)
Africa Kenya 448 67 Municipal ww
in Nairobi UNEP (2014)
Africa Kenya 940 72 Municipal ww
in Nakuru UNEP (2014)
West Asia Jordan 1830 770 150 25 Municipal ww
in Amman FAO (1992)
West Asia General 530 75 15 UNEP (2014)
South Pacific Fiji 450 UNEP (2014)
Central and
South America General 350-450 200-250 25-60 5 -10 UNEP (2014)
Caribbean
Islands General 350-450 200-250 25-60 5 -10 UNEP (2014)
*as Kjeldahl-N; ww = wastewater
2.5.4. Per capita pollution loads
The generally assumed per capita pollution loads for the dimensioning of WWTPs are 60 g BOD5 cap
-1 d-1 and 120 g COD cap-1 d-1 based on data from developed countries (Tchobanoglous et al., 2003).
Campos and vonSperling (1996) however reported that the average household income in Brazil correlates with the per capita BOD5 production. They concluded that the generally adopted text book values based on data from western countries overestimate the per capita organic load production for low to middle income areas in Brazil which were typically below 54 g BOD5 cap-1 d-1. Mara (2003) confirms that the per capita BOD5 load tends to increase with income.
The values proposed by Tchobanoglous et al. (2003) are therefore probably not representative for many of the situations in which DEWATS have to perform. The WHO (WHO/UNEP, 1997) for instance reports that significantly inferior per capita loads may occur (see Table 12). Various authors report per capita BOD5 and COD loads in Africa and Asia which are only half the value valid for western countries.
Henze et al. (1997) compiled information on wastewater characteristics from several countries. They did however not specify which social class is represented or whether the data applies to rural or urban areas. It can be assumed that the values are rather biased towards higher income, urban dwellings:
daily per capita BOD5 load in Brazil and Uganda is 55 g cap-1 d-1 to 70 g cap-1 d-1 and 30 g cap-1 d-1 to 40 g cap-1 d-1 in Egypt and India. The commonly used DEWATS design procedure (Sasse, 1998) suggests a daily per capita BOD5 load 30 g cap-1 d-1 to 65 g cap-1 d-1. In practice DEWATS engineers generally use a per capita load of 60 g BOD5 cap-1 d-1 for their design (personal communication, BORDA).
Table 12: Per capita pollution load values reported for developing and emerging countries Continent/
Region Country Parameters (in g cap-1 d-1) Comments Reference COD BOD5 NH4-N PO4-P
General 70- 150 30- 60 8-12# 1-3 WHO/UNEP (1997)
Africa Morocco 50 Rural areas Abarghaz et al.
(2011)
Africa Kenya 23 UNEP (2014)
Africa Zambia 36 UNEP (2014)
Southern Africa 100 10 2.5 Load to VIP^ UNEP (2014)
West Asia 53 7.5 1.5 UNEP (2014)
West Asia Iran 60 40-45 7-8 0.9-3.7 Peri urban
area Tehran
Miranzadeh (2005);
Rezagholi (1997)
East Asia Thailand 35 Tsuzuki et al. (2007)
East Asia Thailand 81.2 46.4 11.5* 1.9 Peri urban
area Bangkok Tsuzuki et al. (2013)
# as Kjeldahl-nitrogen; * as total nitrogen; ^ Ventilated Improved Pit Latrine (VIP)