Review of frameworks and tools for urban strategic sanitation planning: considering technology innovations and sustainability

To achieve citywide inclusive sanitation in developing countries, a strategic sanitation planning approach (SSA) needs to provide a variety of technical solutions that respond to different urban realities. Despite the development of various SSA frameworks, sanitation planning still often follows a ‘ one-size- ﬁ ts-all ’ approach. Structured decision making (SDM) can help by balancing trade-offs among different solutions. But SDM requires a set of appropriate sanitation options to choose from. Because conventional sewer-based sanitation is often inappropriate, many novel technologies and systems have been developed (e.g. container-based sanitation). While these innovations enhance sustainability, they also increase planning complexity. In this review, we look at available frameworks and tools for SSA and discover a lack of systematic tools for the identi ﬁ cation of planning options that are able to consider the growing portfolio of available solutions and multiple sustainability criteria. Therefore, we critically compare 15 tools from which we compile eight qualities that could help any future tool address the current sanitation challenge: it should be comprehensive, automated to deal with a large number of options, systematic, ﬂ exible towards future innovation and should consider all sustainability dimensions, make a contextualized evaluation, allow for participation, and consider uncertainties to be applicable ex-ante also for novel technologies.


INTRODUCTION
Current technological innovation provides a unique opportunity, especially for developing urban areas, to bypass the unsustainable conventional end-of-pipe approach to sanitation. This has also been recognized in the most recent urban strategic sanitation planning approach: Citywide Inclusive Sanitation (CWIS) ( (Peal et al. b). But most current research focuses on understanding the context or the selection of a preferred option, assuming that a set of appropriate sanitation planning options is already available. Still, every decision support is only as good as the options presented. The current sanitation challenge requires a tool that enables engineers and planners to consider the growing portfolio of technology options and the multiple sustainability criteria when providing suitable planning options.

AIM OF THIS PAPER
In this paper, we attempt to answer three questions: To answer these questions, we start with a description of the current sanitation challenge. Secondly, we provide a review of SSA frameworks and tools that evolved over the past 40 years from an SDM perspective. Thirdly, we systematically compare 15 tools for the identification of sanitation system decision options (step 3 of SDM) in order to evaluate how they address the current sanitation challenge. For this, we are mainly interested in the capability to consider (i) the growing portfolio of available sanitation technologies and system configurations; (ii) multiple sustainability criteria; and (iii) uncertainties related to the local context or novel technologies. 2. Tools to put these concepts into practice: Tools included any decision support for sanitation planning such as 'decision trees', 'computer models', and 'information packages'. Particular attention was given to tools supporting the identification of sanitation system decision options (step 3 of SDM). The literature review revealed a lack of systematic tools that allow for the consideration of the growing number of technology options and decision criteria when generating sanitation decision options (step 3 of SDM). Therefore, we systematically and critically compared 15 tools that can support step 3 of SDM and identified qualities that such a tool should embrace to address the current sanitation challenge.

THE CURRENT SANITATION CHALLENGE
The world has not achieved the Millennium Development Goals (MDG) for sanitation and is not on track to reach the SDG 6.2 (UN ), i.e. sanitation to all. While the MDGs aimed for improved access to toilets, the SDG extended the demand for toilets with a call for looking at entire systems, considering sustainable sanitation, and providing appropriate options for inclusive services.
The idea of appropriate technology initially evolved in the 1970s in order to promote an alternative to the capital-intensive technology of modern industry (Schumacher ). Appropriate sanitation services require a mixture of technologies that are adapted to (i) local skills and materials; (ii) capital resources; (iii) physical conditions, such as topography, soil type, water availability; (iv) and socio-demographic con- The five criteria for sustainable sanitation have been laid out by the Sustainable Sanitation Alliance in 2008 (SuSanA ). To be sustainable sanitation systems must not only provide appropriate technologies that are socially acceptable and institutionally and financially viablethey must also protect the environment by saving/recovering natural resources. Social acceptance here not only refers to user preferences regarding e.g. squatting or sitting or the reuse of human waste but also integrates aspects related to gender issues and the inclusion of marginalized groups.
The definition of sustainable sanitation challenged the sector to shift the focus from end-of-pipe treatment towards approaches that integrate resource recovery and reuse. As cities are responsible for the largest component of global energy, water, and food consumption, as well as related wastewater and organic waste production, this is a promising approach in regard to sustainable development This leads to enhanced transparency and more empirical decision making while taking stakeholder preferences into consideration. The aim is not only to elicit 'better' (more rational) decisions but also more accepted decisions.
Over the past 40 years, various frameworks have been developed for SSA in urban areas. All of them provide a more or less structured framework and/or methodology covering several steps of SDM. And each of them captures a current trend and provides an additional element based on lessons learned from previous efforts (see Table 1  were not prepared to invest the required time and resources for such an approach at that time. The HCES (Eawag ) approach was formulated as guidelines for implementing the Bellagio Principles in Urban Environmental Sanitation Planning (Schertenleib Table 1 | Historical overview of selected strategic sanitation planning frameworks and their contribution to address the current sanitation challenge Combination of bottom-up and top-down approach using a multi-actor and multi-sector approach; prioritization of circular systems that consider waste as a resource and work within different city zones; focus on the enabling environment; not only a framework but a 10-step implementation methodology (iii) cities should be divided into spatial zones (household, neighbourhood, local government, etc.), with systems that emphasize reuse and recycling within these zones in order to solve the problems nearest to where they arise; and (iv) focus should be given to the enabling environment. The HCES approach was tested, piloted, and evaluated in seven cities (Lüthi et al. b). The focus was mainly on community involvement within one zone independently, rather than looking at the zones jointly. Therefore, the scope of citywide sanitation was not achieved as initially intended.
In 2008 (2) understand the existing context and define priorities; (3) develop system options; (4)      Relatively complex yes/no algorithm based on the methods of anal cleansing, population density, affordability, demand for reuse, land availability, soil conditions and user acceptance.  These qualities are not intended as a precondition but are intended to provide guidance to improve the capability of future tools in addressing the current urban sanitation challenge. Moreover, it remains clear that even a tool with all these qualities will not provide the solution to the sanitation crisis but will only provide one piece of the puzzle. Any planning process depends first and foremost on political will and local leadership backed up with sufficient time and human and financial resources for strategic sanitation planning.

TOOLS TO MAKE PROCESS GUIDES OPERATIONAL
The greatest challenge in sanitation planning lies in institutions laying out the responsibilities and resources for developing and implementing urban sanitation plans.

LIMITATIONS
This paper is based on the analysis of a broad range of literature that was collected over several years while paying attention to comprehensiveness. However, there is a strong risk for cognitive bias related to the GRASP project and the experts involved. There are, of course, other resources for supporting SSA. Because of this, there is a limitation in the empirical data, underlying the conclusions, giving it a speculative character. Nevertheless, despite these limitations, we feel that the identified research challenges are generalizable for the sanitation sector and can provide guidance for more effective strategic planning and SDM for urban sanitation in the future.

CONCLUSIONS
This paper provides an overview of the current sanitation challenge and existing strategic sanitation planning frameworks from a structured decision-making (SDM) perspective. It seeks to identify research needs for making strategic sanitation planning operational in practice.
The paper looks in detail at one need which is the availability of systematic tools, or lack thereof, for the identification of sanitation system planning options ( Better decision-making and planning approaches are urgently needed to achieve safely managed sanitation in the rapidly developing cities of the global South. Providing sanitation options that are appropriate for the different areas within a city that can deal with current challenges related to population growth, climate change, and resource depletion is an important step towards more citywide inclusive sanitation planning. However, it is only one piece of the puzzle and needs to come with political will and human and financial resources not only for planning but also for implementation and maintenance in the long term.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.