Nutrient recycling from sanitation and energy systems to the agroecosystem - Ecological research on case studies in Karagwe, Tanzania [original]

Vol. 10(43), pp. 4039-4052, 22 October, 2015

DOI: 10.5897/AJAR2015.10102

Article Number: EBC13E356011

ISSN 1991-637X

Author(s) retain the copyright of this article

http://www.academicjournals.org/AJAR

African Journal of Agricultural

Research

Full Length Research Paper

Nutrient recycling from sanitation and energy systems

to the agroecosystem- Ecological research on case

studies in Karagwe, Tanzania

A. Krause1,3*, M. Kaupenjohann2, E. George3 and J. Koeppel4

1Microenergy Systems―, Center for Technology and Society, Technische Universität (TU) Berlin, Germany.

2Department of Ecology, Department of Soil Science, TU Berlin, Germany.

3Leibniz Institute of Vegetable and Ornamental Crops (IGZ), Großbeeren, Germany.

4Environmental Assessment and Planning Research Group, TU Berlin, Germany.

Received 3 July, 2015; Accepted 25 September, 2015

Open cycles of organic carbon and nutrients cause soil degradation. Procedures such as ecological

sanitation (EcoSan), bioenergy and Terra Preta practice (TPP) can contribute to closing nutrient cycles

and may, in addition, sequester carbon. This paper introduces three projects in Karagwe, Tanzania, and

their applied approach of integrated resource management to capture carbon and nutrients from

different waste flows. Substrates derived from these case studies, biogas slurry, compost and CaSa-

compost (containing biochar and sanitized human excreta), were assessed for their nutrient content by

analysis of the total element composition. Evaluation focused on potential impacts of the tested

amendments on the nutrient availability in the soil as well as on the local soil nutrient balance. Results

revealed that all substrates show appropriate fertilizing potential compared to literature, especially for

phosphorus (P). CaSa-compost was outstanding, with a total P concentration of 1.7 g dm-3 compared to

0.5 and 0.3 g dm-3 in compost and biogas slurry respectively. Furthermore, these soil amendments may

reduce acidity of the soil, with a calculated liming effect of 3.4, 2.6 and 7.8 kg CaO for each kg of

nitrogen added for biogas slurry, compost and CaSa-compost respectively. To offset negative P

balances in Karagwe, about 8100, 6000 and 1600 dm3 ha-1 are required for biogas slurry, compost and

CaSa-compost respectively. We conclude that especially CaSa-compost might offer immediate positive

effects to crop production and nutrient availability in the soil.

Key words: Ecological sanitation, bioenergy, Terra Preta practice, biochar, biogas slurry, compost, soil

amendments, soil improvement, waste as resource.

INTRODUCTION

Open cycles cause agronomic problems

Since more nutrients are taken out of the agroecosystem

than are put back, anthropogenic activities create open

cycles of mineral nutrients and carbon (C) (Lal, 2006).

Such activities comprise among others: Excessive

*Corresponding author. E-mail: [email protected].

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4040 Afr. J. Agric. Res.

deforestation for firewood, exploitation of phosphate

rocks for fertilizer production, and energy consumption for

production of synthetic fertilizers. Furthermore, most

current sanitation systems waste nutrients from human

excreta (especially nitrogen (N), phosphorus (P) and

potassium (K) as well as micronutrients) since they are

either disposed in the ground (pit latrine, ashes of

incinerated sewage sludge) or enter the aquatic system

(pit latrine, flush toilet), where they cause eutrophication

and lead to contamination of the groundwater with fecal

microorganisms(Esrey et al., 2001;Graham and Polizzott

o, 2012; Meinzinger, 2010). In general, open cycles can

cause soil degradation and loss of soil fertility since

cultivated soils become increasingly deficient in essential

plant nutrients when long term cropping takes place

without replacement of nutrients (Hartemink and

Bridges, 1995). In addition, soil organic matter (SOM),

which is the major building block of a fertile soil, might be

depleted by continuous cropping if the plant residues are

not put back into the soil after harvesting

(Batjes and Sombroek, 1997). Consequently, the soil

might show declining water and nutrient retention

capacity and an increasing tendency to soil erosion

(Horn et al., 2010). Tropical climate conditions aggravate

such soil degradation; with year-round elevated

temperature, SOM is lost due to fast microbial

decomposition of organic matter; heavy rains during the

rainy season in turn cause leaching of mineral nutrients

(Lal, 2009). It is widely agreed that in order to secure

sustainable food supply for everyone, soil degradation

must be reversed and soil productivity enhanced.

Problems of using synthetic fertilizers in Sub-

Saharan Africa

Agricultural practices using synthetic fertilizers often add

too much N to the soil and sometimes neglect input of P,

K and micronutrients, which can result in imbalanced

plant nutrition (Lal, 2009). Furthermore, nutrients added

by synthetic fertilizers often are immediately available

and thus can be subject to high losses via leaching and

volatilization (Finck, 2007; Savci, 2012). Moreover, the

International Assessment of Agricultural Knowledge,

Science and Technology for Development (IAASTD)

showed that in some parts of Sub-Saharan Africa (SSA),

especially poor farmers do not have access to synthetic

fertilizers (Markwei et al., 2008). Those who have access

often lack adequate information on their appropriate use

(ibid.). Inappropriate use of synthetic fertilizers, however,

may result in soil acidification, pollution of water bodies,

and emissions with global warming potential to the

atmosphere (Markwei et al., 2008; Savci, 2012).

Furthermore, the production of synthetic fertilizers

requires energy; for example about one third of the total

energy input to crop production of the United States of

America is required to produce, to package, to transport

and to apply synthetic fertilizers (Gellings and Parmenter,

2004).

Solutions based on using locally available organic

fertilizers

Kiers et al. (2008) concluded that in African countries

reversing soil infertility might be achieved ―through the

use of locally available resources‖, because the use of

synthetic fertilizers is not a feasible option for many

subsistence farmers. In ―Agriculture at a crossroads―

McIntyre et al. (2009) called for a focus on efficient,

small-scale agroecosystems with almost closed nutrient

cycles. In addition, the IAASTD demanded that research

in a SSA context should reorient ―towards integrated

nutrient management approaches‖ (Markwei et al., 2008).

Kimetu et al. (2004) demonstrated in Western Kenya that

―inorganic N additions can be fully substituted by organic

N additions if the appropriate source of organic matter is

applied‖. Furthermore, the intensified use of organic

fertilizer can reduce the cost of fertilization in crop

production in SSA (Markwei et al., 2008).

In order to create positive C and nutrient budgets, SOM

can be enhanced through addition of organic

amendments, as Lal (2009) pointed out. He further

suggested that both organic residues, such as compost

and animal manures, and biological N-fixation should be

included in the nutrient management (ibid.).

Stoorvogel (1993) particularly emphasized the efficient

use of organic household waste as a means to supply

nutrients. Beardsley (2011) pointed out that human

excreta ―is an abundant but often ignored source of P

available for recycling worldwide‖. Another important soil

management practice to strengthen the nutrient cycling

process in SSA is acidity management through liming, as

described, for example, by Batjes and Sombroek (1997).

Approaches towards closing the loop

In our research, we focus on the following practices for

local nutrient and C recycling: (1) Composting in general,

as well as co-composting of human excreta and

ecological sanitation (EcoSan); (2) Provision of bioenergy

combined with agricultural use of residues; (3) Terra

Preta practices (TPP) – using biochar as a soil

amendment.

Composting and ecological sanitation

Composting is a globally common method in agriculture

whereby organic residues are mixed with mineral

components and subsequently aerobically decomposed

by macro- and microorganisms (for East-Africa see work

of e.g. Amoding et al. (2005), Karungi et al. (2010) and

Krause et al. 4041

Figure 1. Relationship between temperature and time required to inactivate certain pathogens (according to

Feachem et al., 1983, graphic adopted from Vögeli et al., 2014; corresponding combinations of time and

temperature for the described possible treatments are indicated)

Tumuhairwe et al. (2009). EcoSan facilitates co-

composting of human excreta as an alternative to

conventional sanitation systems. EcoSan aims at (i)

―closing the loop‖ by recycling nutrients from

humanexcreta in order to improve soil fertility; (ii)

avoiding potential human health risks by sanitizing urine

and feces; (iii) preventing the pollution of freshwater and

marine environments by avoiding waste water discharge

into natural water bodies (Winblad et al., 2004). Further

benefits of EcoSan, according to Esrey et al. (2001), are

that it is: (i) A decentralized system based on household

and community management and, thus, omits investment

in large-scale infrastructure; (ii) Particularly appropriate in

areas with water shortages or irregular water supply

since no or very little water is required; (iii) Feasible in

both rural and urban areas as well as for rich and poor

people alike. Usually, urine and feces are stored and

processed on-site. A number of different types of

composting toilets are in use in EcoSan, e.g. the urine

diverting dry toilet (UDDT), which collects human excreta

separately (see Morgan, 2007, for further description and

discussion of ―Toilets That Make Compost - Low-cost,

sanitary toilets that produce valuable compost for crops in

an African context‖). According to the World Health

Organization (WHO, 2006) urine is safe for use as a

fertilizer, untreated or after short storage. However, feces

mostly contain pathogens (such as viruses, bacteria and

worm eggs) and require treatment (ibid.). Techniques for

sanitation include: dehydration or drying, e.g. through

UDDT with a separation of the solid parts and the liquid

fraction of the excreta and improved ventilation system

(Winblad et al., 2004); disinfection by using additives, e.g.

urea (Vinnerås, 2002) or lactic acid bacteria

(Factura et al., 2010); disinfection through exposure to

elevated temperatures over time, e.g. mesophilic or

thermophilic composting (Niwagaba et al., 2009;

Ogwang et al., 2012) or pasteurization (RKI, 2013;

Schönning and Stenström, 2004). In general, thermal

sanitation relies on a temperature/time relationship to

inactivate certain pathogens, as described by

Feachem et al. (1983) (Figure 1).

Currently, there are no national regulations for the

treatment of human excreta, in neither Tanzania nor

Germany, but different guidelines for thermophilic

composting exist. The WHO (2006) recommended a

treatment at 55 to 60°C over several days up to one

month depending on the conditions (e.g. constant control

of the temperature). In Germany, the following thermal

treatments are required for organic waste in general:

55°C for two weeks, 60°C for six days or 65°C for three

days (German BO, 2013).

Bioenergy and the agricultural use of its residues

Bioenergy technologies focus on energy recovery from

biomass. Also, by-products and residues from bioenergy

provision can be recycled back into the agroecosystem.

The main principle is the conversion of biomass to heat

for either the consecutive production of electricity or

4042 Afr. J. Agric. Res.

direct provision for productive processes (e.g. for a

bakery, green-house heating) and consumption in

households or institutions (e.g. for cooking and heating)

(Kaltschmitt et al., 2009). In this study, our focus is

onprovision of cooking energy at household level and the

applied technologies include: three stone fire, charcoal

burner, microgasifier and a system using a biogas

digester and biogas burner. The use of firewood, three

stone fires and charcoal burners is currently most

common in many countries of SSA. Ash is the main

residue from these bioenergy applications and contains

mineral nutrients such as P and K as well as calcium (Ca)

and magnesium (Mg), but hardly any C, N or sulphur (S)

since these elements volatilize during the oxidation

process. Ash is therefore often used as a soil amendment

or addition to compost. Another small-scale technology is

the biogas digester, which is used for cooking both in

households and institutions, such as schools or hospitals

(Vögeli et al., 2014). Organic wastes are anaerobically

digested via microbiological activity in a closed fermenter,

resulting in a methane-rich combustible gas as the main

product and biogas slurry as a liquid residue (ibid.).

Small-scale and low-tech biogas digesters usually

operate in a mesophilic range of about 30 to 40°C and a

retention time of around 40 days

(Kossmann et al., undated). Biogas is accumulated inside

the digester or in a separate storage tank and is usually

combusted in a biogas burner. Biogas slurry can be used

as a fertilizer since it contains most of the mineral

nutrients from the digested organic waste in an already

plant-available form (Vögeli et al., 2014). Caution and

additional treatment of the biogas slurry is required,

however, in case human excreta is also digested since

pathogens are not inactivated under the mesophilic

conditions mentioned above (Figure 1). In Nepal, for

example, Lohri et al. (2010) showed that the biogas slurry

from mixed fermentation of human excreta and kitchen

waste contained pathogens such as helminth eggs.

Moreover, inappropriate use of the liquid biogas slurry

can cause eutrophication if it is applied in excess or

discharged directly to a receiving body of water

(Kossmann et al., undated). Finally, households can meet

their energy demand by using microgasifiers, which are

improved cooking stoves that use dry biomass and

spatially separate the transformation of biomass into

combustible wood-gas from the subsequent oxidation of

the gas (Mukunda et al., 2010; Roth, 2013). One

particularly prominent stove design is called the TLUD

(―Top-Lit Up Draft‖), which is licensed as an open source

technology (Anderson and Reed, 2007). Apart from heat,

the stove provides charcoal of about 10 to 30% of the fuel

fresh weight as a by-product (Roth, 2013). As for ash,

charcoal preserves mineral nutrients. It also contains C in

a concentration of about 60 to 75% of its dry matter (DM)

(McLaughlin et al., 2009). The charcoal can be used for

further provision of energy by directly pouring the hot

charcoal onto a conventional charcoal burner, to continue

cooking immediately, or by making charcoal briquettes in

a separate process with an accumulated amount of

charcoal. Charcoal can also be used as a soil

amendment, which is then termed biochar (Taylor,

2010). Altogether, residues from bioenergy processes

have a potential for use as soil amendments; however,

their quality depends on the composition of the feedstock

used and the application practice. There is a need for

field experiments to evaluate the impact of biogas slurry

on the local carbon balance as well as on soil

characteristics and productivity (Bogdanski and di

Caracalla, 2011). The positive effects of pyrolitic charcoal

as a soil amendment are historically evident in findings of

Terra Preta soils, which we will introduce in the following

section. However, there is still a lack of scientifically

rigorous field experiments using biochar derived from

microgasifiers on tropical soils.

Terra Preta practices (TPP) - using biochar as a soil

amendment

One particularly interesting and promising holistic

approach for improving or remediating degraded soils is

the principle of ―Terra Preta‖ (Portuguese for ―Black Soil‖

= ‖Udongo Meusi‖ in Swahili), as practiced by people in

the Amazon basin in Brazil, South America, centuries ago

(Sombroek, 1966; Glaser et al., 2002). Lehmann et

al. (2003b) classified Terra Preta as Anthrosol, a human-

made, fertile, black soil. Glaser and Birk (2012) found that

it mainly contains charcoal, animal and human excreta as

well as other organic and inorganic wastes. Compared to

surrounding soils, including Ferralsol, Acrisol or Arenosol,

the Terra Preta soils show significantly higher availability

of P, Ca, manganese (Mn), and zinc (Zn)

(Lehmann et al., 2003a). For example,

Falcão et al. (2009) found up to 40 times larger

concentrations of plant-available P in Terra Preta than in

surrounding natural soils. Other characteristics include

high water and nutrient retention capacity as well as a pH

of around 5.7, adequate for plant growth

(Lehmann et al., 2003a; Horn et al., 2010). Biochar plays

a major role for the specific properties of Terra Preta

because it builds up a stable stock of SOM. Biochar

shows an aromatic C structure with many micro pores,

large surface, high adsorption capacity and a C-

concentration of about 70 to 80% of DM

(Lehmann and Joseph, 2009). In some soils, biochar can

significantly improve the availability of both nutrients and

water by effecting chemical and hydraulic characteristics

of the soil. It can also positively affect the activities of soil

microbial communities (Lehmann and Joseph, 2009;

Glaser and Birk, 2012). According to Taylor (2010),

biochar works as a catalyst in the soil, because it

―facilitates reaction beneficial to soil dynamics without

being consumed in the process‖. This means that much

of the biochar persists in the soil and is not decomposed

in the way many other organic materials are (ibid.).

Therefore, biochar amendments may enhance plant

growth in some cases, although nutrient inputs from

biochar are low (Lehmann and Joseph, 2009).

Consequently, its application was tested in

combination with mineral fertilizers (Kimetu et al., 2004;

Jeffery et al., 2011), in combination with compost that

releases nutrients over time (Liu et al., 2012;

Schulz et al., 2013), and as compost-additive to be

enriched and loaded with nutrients during the composting

process (Kammann et al., 2015).

Recently, Frausin et al. (2014) revealed the presence

of so-called African Dark Earth at more than 134

locations in several West-African countries including

Liberia, Sierra Leone, Guinea and Ghana. This Terra

Preta-like African Anthrosol is preferably located in the

vicinity of towns and mainly is the product of women

doing appropriate management of wastes from housing

and farming (ibid.). Altogether, TPP - using biochar as

compost-additive and soil amendment - is seen as a

―suitable technique helping to refine farm-scale nutrient

cycles‖ (Schulz et al., 2013).

Research objectives

Based on the context described in the introduction, we

hypothesize that new approaches which combine

EcoSan, bioenergy and TPP can contribute to soil

improvement and resource protection by recycling of

nutrients and C, if sanitation is taken into account and

integrated appropriately. Especially the use of biogas

slurry from fermentation of organic waste as a fertilizer

and the combined composting of residues from

microgasification and sanitized human excreta are

promising methods. However, there is need for practice-

oriented experiments and assessment of the local

ecological impacts under the specific conditions of

tropical regions. Hence, the objectives of this paper were

(i) to introduce three case studies from Karagwe,

Tanzania, and their applied approach of integrated

resource management; (ii) to assess the substrates

derived from these projects with respect to their nutrient

concentrations; and (iii) to evaluate potential impacts of

the tested amendments on the nutrient availability in the

soil as well as on the local soil nutrient balance.

MATERIALS AND METHODS

Farming activities in Karagwe, Tanzania

Karagwe district is located in Kagera region in northwest Tanzania,

a hilly area situated at an altitude of about 1200 m up to

1800 m.a.s.l., semi-arid with equatorial-tropical climate

(Baijuka and de Steenhuijsen Piters, 1998). The average daily

temperature is about 21°C, with a range from 10°C at night to

> 40°C during the daytime (Blösch, 2008). Rainfall is bimodal with

rainy seasons from March to May (long rainy season) and October

to November (short rainy season), with crop cultivation taking place

Krause et al. 4043

during both seasons (Tanzania, 2012). Precipitation ranges

between 1000 and 2100 mm a-1, with annual and regional

differences (Blösch, 2008).

According to the national sample census of agriculture

2007/2008 for Kagera region, most families in Karagwe

districtsubsist on farming activities (Tanzania, 2012): about 45% of

the population work full-time on their farms and more than 86% of

the households sell agricultural products grown on their farms. On

average, around 0.75 ha usable land is available per household out

of which around 83% is planted. The most important permanent

crops are banana and coffee, while beans, sorghum and maize

dominate annual cropping. Most of the planted land is used multiply

in mixed cropping systems and only some 16% of the land is used

for temporary mono-cultural cultivation. A majority of approximately

78% of the farmers in Kagera region who apply fertilizers on their

land, use organic fertilizers which are according to

Baijukya and de Steenhuijsen Piters (1998) mainly grasses (mulch)

and farmyard manure. However, the supplied amount only suffices

for roughly 5% of the planted land (distributed to 0.7 and 4.3 %. of

the planted land in the long and short rainy season respectively).

Synthetic fertilizers are used on less than 1% of the planted land in

Kagera region. In 2010 we conducted a preliminary study in

Karagwe district including a survey on 10 households and soil

sampling at three different farms. We found that small-scale farmers

in Karagwe live on an average with six people in one hous

d. In

addition, we found that some major problems of local agriculture

are a very low soil pH of 3.8 to 4.2, low nutrient availability

(especially P) and soil erosion due to a hilly landscape. Concerning

sanitation services, a majority of more than 90% of the rural

population of Karagwe district use pit latrines, around 6% do not

have any toilet so use bushes and only 1% uses flush toilets in

combination with septic tanks (Tanzania, 2012). Hence, for 91% of

rural households in Karagwe district, excreta are disposed in a pit

or tank after dropping without any treatment or use. Concerning

energy supply, the most common source of energy for cooking is

biomass, with about 96% of the rural households using firewood

and 3% using charcoal (Tanzania, 2012). It is common in Karagwe

to add ashes from three stone fires to the compost.

Grassroots projects in Karagwe realizing integrated resource

management

Since 2008, two local non-governmental organizations, namely

MAVUNO Project Improvement for Community Relief and Services

(MAVUNO; meaning ―harvest‖ in Swahili) and CHEMA Programme

for Community Habitat Environmental Management (CHEMA), have

initiated projects in cooperation with the German association

Ingenieure ohne Grenzen e.V. (Engineers Without Borders, EWB)

and Technische Universität (TU) Berlin. These projects follow a

community-participatory approach to appropriate development of

technologies and aim at resource protection, autonomous energy

supply and safe sanitation services. Together, these projects

present an integrated approach to resource management as well as

recycling of nutrients and C (Figure 2). Their process combines

three systems: The energy system, whereby cooking energy is

provided as heat by either burning biogas from a small-scale biogas

digester or by microgasifiers; the sanitation system based on

EcoSan; finally, the recycling of by-products from both systems,

namely biogas slurry, biochar and sanitized human excreta, back

into the agroecosystem. In the latter, composting and the principles

of TPP are applied to capture nutrients and C from different waste

flows.

One of the expected results is soil improvement, to ensure long-

term food security and income generation for the rural population.

The respective technologies were developed and tested in Karagwe

within three pilot projects:

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