Appropriate tree types for reusing treated wastewater in Dar es Salaam, Tanzania

Agricultural Techniques Agroforestry
#1

Dear All!

I’m currently working in a project in Dar es Salaam, where we are implementing a Faecal Sludge Treatment Plant (10m3/d) in a community (Barafu, Mburahati):

The treated wastewater will be used for irrigating a tree line at the edge of a soccer field. The trees shall provide shadow and ideally also have additional benefits, such as flowers (decorative) or fruits. The challenge is that the trees will be irrigated by a sub-surface irrigation system (PVC-Pipes). Thus, there is a risk that the tree roots grow into the irrigation pipes and cause damage. The irrigation pipes are at 30 – 60 cm below the ground level and end at 40 cm away from the place where the tree shall be planted.

My question: Which tree types can be recommended that fit the named requirements, with roots that mainly grow vertical and deep.

Thanks in advance for your experiences/recommendations/ideas!

#2

The only idea I can think of is using a clumping type variety of bamboo. Most of them grow vertical roots. It isn’t a tree per se, but maybe it works for you. When you find a type you like, double check to make sure the roots grow only vertical. I checked out Guadua, one of the largest and most popular clumping varieties and it won’t work. It spreads horizontally too much. Maybe a smaller variety is better. Something used for furniture making.

Hope this helps.

#3

Here is a response shared by ECHO East Africa Director Erwin Kinsey - there are good people in the Tanzania Forestry institutions - one who has been helpful to me in the past is Frank Mbago mbagomgalla@gmail.com.

I found this article online, but one needs to purchase it if you are not on a university faculty! https://www.sciencedirect.com/science/article/pii/037811279190245Q

We have used a French drain for our sewage water since establishing our septic system in 1996, and it is still working. It has 10cm perforated sewage pipes laid on sand and buried, with bananas planted over them. I am sure species is important, and I will keep looking for more information for you, to know the growth characteristics of roots of local and exotic trees. Grevillea robusta is well known for its roots NOT encroaching onto crops planted nearby, and thus, a good agroforestry species. I believe that Acacia albida, now known as Faidherbia albida, is one indigenous tree which also has the same reputation, besides also being ideal for providing shade during the dry season and losing its leaves during the wet season in unimodal rainfall areas.

Kind regards,
EK

#4

Hi Tim,

This is ECHO’s plant information sheet on Strawberry Tree. It has basic information about the tree. I noticed it commonly growing in Dar on my last visit there. It is grown along roadsides, and is often used as a shade tree over ornamental/tree nurseries. It seems to have a spreading growth habit there. In our experience from Florida, strawberry tree is not a long-lived tree. This may be due to our high water table, which prevents deep root growth. We usually expect about 5 years before the tree starts to decline. I wonder if the frequent water and wet conditions of your drain field will have a similar effect.

I would suggest, similar to what Erwin was saying, that a shallow rooted species might prove to be the best option for you. Trees that are normally deep rooted may not develop deep roots in the presence of ample water. Many tree species demonstrate the ability to adapt their root growth to avoid competition for water resources, or (as in this case) take advantage of ample water resources. So, you may plant a ‘deep rooted’ tree and find that it is not developing roots in the expected way.

Banana is probably not going to meet you requirements for a long term shade tree. You could also look into whether coconut palm or oil palm could be an option. Most of the roots of these palms are going to be found in the upper 1 meter of soil. This might require placing your pipes at a deeper depth or mounding soil where the palms would be planted. This may also help with any water logging issues.

I have a few documents that you might find helpful. I will try to send those to you by email.

I hope this is helpful,

Tim Watkins

#5

This is an email stream discussing Tim’s question. It may be of value to others with similar concerns.

Dear Erwin and Tim:

Thanks for copying me on this, Erwin. Tim, I don’t have any immediate suggestions of tree species to use for reuse of treated wastewater in Dar, but I would urge you to look into suitable indigenous species rather than the exotic “usual suspects” such as Moringa, Neem, Avocado, Cashew, and Guava. I would particularly avoid Neem because of its horribly aggressive nature, growing colonially and crowding all native species over wide areas. For positive suggestions I refer you to my friend and colleague Anne Outwater (copied here), who has vast experience with native coastal species suitable for various applications. I’ll be working with Anne tomorrow and will ask her opinion about this.

Best wishes,
Roy

Roy E. Gereau
Tanzania Program Director
Africa and Madagascar Department
Missouri Botanical Garden
4344 Shaw Blvd.
St. Louis, MO 63110
U.S.A.

Dear Erwin, dear Roy,

Thanks for your informative responds!

Another species I came across is Panamaberry (Strawberry Tree/Jamaican Cherry). It would fit our requirements, but we are not sure about its root structures. Do you have any experience with this species?

Best regards,
Tim

Dear Tim:

Panama Berry/Jamaican Cherry/ Strawberry Tree are all vernacular names for Muntingia calabura in the plant family Muntingiaceae or Elaeocarpaceae, a species native to Central and South America. It is a very popular and apparently drought-hardy street tree around Dar and other places on the coast, and a seed scientist at the Tanzania Tree Seed Agency in Morogoro has recently informed me that under some circumstances it can be seriously invasive, crowding out and replacing native species. Given that report and the easy dispersal of its seeds by birds, I would be very hesitant to recommend its use for this purpose. Let’s please try to find some beneficial native species instead!

Best wishes,
Roy

It seems like you are talking about two things? - sludge, and waste water? sludge should be heated and tested for helminths after some months or year, and then can be spread on crops that are not eaten raw.

What is usually used here in Tanzania for treating pre-treated wastewater is constructed wetlands planted with Phragmaites mauritanius or Typha. sp - both native. They do very well if maintained (light maintenance). Also research is being done with various mangrove species.

There is a fairly strong research program at UDSM at CoET. And if you are in Arusha you could speak to the current Vice Chancellor at Nelson Mandela, Professor Njau - who is an engineer who has most of his career on constructed wetlands with those above species.

But also, in the system you describe, I doubt whether species is important. You could plant a native timber tree (mvule/ Milicia excelsa would work for you )and get rich in 10-15 years as well as provide habitat for native fauna, or plant some fruit trees and improve your nutrition! (oranges grow very well here - avocado needs a cooler climate).

You don’t really need a drought hardy tree - unless the waste water is seasonal - all trees native to the coast will be sufficiently drought hardy.

Anne H. Outwater PhD, RN

Head, Department of Community Health Nursing, Muhimbili University of Health and Allied Sciences
World Learning Advancing Leaders Fellow, PO Box 105211, Msasani, Dar es Salaam, Tanzania. Phone 255 (0)713 856962

Dear Roy and Dear Anne,

I appreciate your advice of promoting native, not invasive species! Milicia excels looks great, but also here I’m afraid it will interfere with the sub-surface pipe system, so maybe better to go for a smaller tree, as orange.

Concerning the treatment system: Yes you are right, faecal sludge and wastewater are two things, nevertheless in Dar es Salaam (and many other regions) faecal sludge form pit latrines and septic tanks contains a lot of water. We developed and are implementing the DEWATS approach (modular approach of low cost/easy to construct and operate technologies) for wastewater, but also faecal sludge treatment (modules are e.g. Biogas Reactor, Anaerobic Baffled Reactor, Anaerobic Filter, Planted Gravel Filter). The sludge coming from these systems is already quite stabilized, but in addition it is dried in Sludge Drying Beds. The treated wastewater (also the one from faecal sludge) can be reused for irrigation. Usually we use the water to irrigate Bananas. Some disadvantages of constructed wetlands are the huge area they require, low treatment efficiency and the methane they produce (climate gas). Thanks also for reminding me of Professor Njau, we are in close contact with him for several other projects, but I’ll call him for the current challenge.

Thanks for all your helpful inputs!

Best regards,
Tim

Anne Outwater

to Erwin, Roy, fettback, Fettback, Frank, me, Bob

What you are doing sounds very interesting!

Constructed wetlands: huge area? I guess compared with what. They do need pre-treated wastewater… so maybe including the Waste Stablization Ponds or the septic tank.

low treatment efficiency? - I guess compared with what? We are finding that it is quite good compared to WSP alone.

Methane: I didn’t know about that! I wonder from what biological action.

I have never had Milicia do anything strange with its roots - my feeling is they go down quickly looking for water. +Baphia kirkii is another one which is very root friendly, shady, beautiful, rare, fast growing to a medium size, and only found on this Swahili coast. I can give you seedlings if you like.

Best wishes,

Hello,

Yes you are right: the mentioned disadvantages are depending on the technology you compare it with. And yes, constructed wetlands don’t produce methane, but if they are used as a secondary treatment after constructed wetlands, or septic tanks methane is produced in the primary treatment. Even the PGF we recommend is basically a horizontal constructed wetland, but the pre-treatment with BD, ABR and AF is more effective and climate friendly than septic tanks and WSPs. In the systems we are currently implementing we are not including the PGF because it is an additional area that is used, more implementation costs, more O&M requirement, and there is no need of reducing plant nutrients when using the treated water for irrigation.

If you are interested in seeing the systems, welcome to BORDA in Mikocheni!

Best regards,
Tim

Dear All,

Thanks again for all your inputs! I think Baphia kirkii, Milicia excels, Orange Trees and Palm Trees are then the pre-selected group.

Best regards!
Tim