4.4.4 Planning Effluent Management

The chapter offers a rationale for managing the effluent and illustrates why post-treatment is required.

When determining how to manage the effluent, the global WASH cluster’s “Sanitation Quality Standards for Emergencies” offers guidance.

Based on the decision tree provided, it has been concluded that the effluent can be infiltrated as all the required criteria have been met. However, to prevent the drain fields from clogging too quickly and to ensure that the recommended TSS level of less than 200mg/L is achieved, additional treatment steps such as Anaerobic Baffled Reactor (ABR) and Anaerobic Filter (AF) are implemented. 

Let’s determine if achieving the necessary TSS is possible by implementing all proposed treatment steps and if the resulting effluent would be safe for irrigating vegetable gardens.

The table below outlines the characteristics of the Influent (excreta and flush water) identified in the characterisation study.

Parameter IN	Median Value	Unit	Source
TSS	7,900	mg/L	Characterization study
COD	21,200	mg/L	Characterization study
TN	2,200	mg/L	Characterization study
E.coli	500,000	CFU/100mL	Characterization study

First Treatment Step: Biogas Digester

To predict the quality of the effluent, first, the typical treatment performance for the treatment unit needs to be understood – in our case, a fixed dome digester. The following performance values were collected from the literature review, as presented in the table below. Single values are used instead of a range to simplify the exercise objective.

Parameter	Value	Unit	Source
TSS	60	%	Literature
COD	70	%	Literature
TN	35	%	Literature
E.coli	90	%	Literature

We use a formula to predict the quality of the effluent by subtracting the reduced concentration from the influent concentration of each parameter, using the formula presented below.

Where,

Parameter	Description	Unit
COUT	Concentration of effluent	mg/L or CFU/100mL
CIN	Concentration of influent	mg/L or CFU/100mL
RR	Reduction rate	%

Let’s apply the formula for predicting the TSS concentrations for the effluent.

When we apply the same formula for the remaining parameters, we get the following results.

Parameter	Value	Unit	Source
TSS	3,160	mg/L	Calculated
COD	6,360	mg/L	Calculated
TN	1,430	mg/L	Calculated
E.coli	50,000	CFU/100mL	Calculated

The WHO standards for E.coli <1000 CFU/100mL  are not met for safe use.

Second Treatment Step: Anaerobic Baffled Reactor

Let’s calculate the anticipated quality of the effluent following ABR treatment based on the performance metrics outlined in the following table.

Is additional treatment with AF still required?

Parameter	Value	Unit	Source
TSS	70	%	Literature
COD	80	%	Literature
TN	15	%	Literature
E.coli	80	%	Literature

Applying the same calculation methodology as above, we get the following results.

Parameter	Range	Unit	Source
TSS	950	mg/L	Calculated
COD	1,275	mg/L	Calculated
TN	1,215	mg/L	Calculated
E.coli	10,000	CFU/100mL	Calculated

The effluent quality does not meet the WHO standards and further treatment by AF is needed.

Third Treatment Step: Anaerobic Filter

Let’s calculate the anticipated quality of the effluent following AF treatment based on the performance metrics outlined in the table.

Is additional treatment required and is the effluent safe to use for irrigation?

Parameter	Range	Unit	Source
TSS	60	%	Literature
COD	80	%	Literature
TN	20	%	Literature
E.coli	80	%	Literature

Applying the same calculation methodology as for the previous treatment steps, we get the following results.

Parameter	Range	Unit	Source	WHO Effluent Standards
TSS	190	mg/L	Calculated	<200
COD	254	mg/L	Calculated	<250
TN	972	mg/L	Calculated	<25
E.coli	2,000	CFU/100mL	Calculated	<1,000

The set treatment objective is only partially met (TSS), thus further treatment is required. The treated wastewater is not yet safe to use for the irrigation of low-risk crops.