Multidimensional Assessment of Biojardineras (Nature-Based Solutions) for Decentralized Greywater Treatment in Costa Rica

Introduction

  • Latin America (LATAM) possesses water availability per capita ≈ 4\times the world average; yet only 46\% of domestic wastewater is safely treated.
  • Costa Rica: \approx 94\% potable‐water coverage vs. only 14\% population served by sewerage with treatment; additional 8\% served by sewers without treatment; remaining users rely on septic tanks.
  • Common practice: blackwater → septic tanks; greywater → stormwater gutters → rivers (esp. in Great Metropolitan Area, GAM) causing severe pollution of Tárcoles River watershed (hosts 55\% of national population & 80\% of industry).
  • Retrofitting centralized sewers plants is socio-economic & technical challenging.
  • Nature-Based Solutions (NbS) such as Constructed Wetlands (CW, locally ‘Biojardineras’) offer low-cost, low-maintenance, decentralized options and potential for leapfrogging toward sustainable sanitation.
  • Key implementation hurdles: functional uncertainties, policy gaps, financial constraints, space limitations, silo mentality among stakeholders, negative public perceptions (odour, mosquitoes, etc.).

Research Goals

  • Multidimensional assessment (technical, socio-economic, political-regulatory) of NbS for decentralized greywater treatment in Costa Rica.
  • Identify gaps & requirements for national upscaling; share transferable insights for other LATAM regions.

Methodological Framework

  • Real-World Lab (RWL) established in Llorente, Flores, within Quebrada Seca – Río Burío watershed (GAM).
    • Provides physical & societal context; supports transdisciplinary experimentation & stakeholder learning.
  • Prototyping: several small-scale HSSFCW (‘Biojardineras’) built; one long-term operational prototype at CNFL “Finca de la Sostenibilidad y la Energía” selected for intensive monitoring.
  • Dimensions & Tools
    • Technical: water-quality monitoring (10 campaigns May–Nov 2022); literature review of other Biojardineras.
    • Socio-economic: document review (45 regs & studies), 5 municipal interviews, 7 expert interviews, 154 household surveys, field observations & videos.
    • Political-regulatory: 3 focus groups (13 experts, multi-level & sectors) analysing legal feasibility for private vs. public installations.

Technical Findings

Prototype Description (CNFL)

  • Geometry: width 1.8\,\text{m}, length 3.3\,\text{m}, depth 1\,\text{m}; lined with geomembrane; filled with 25\,\text{mm} pebbles; planted with endemic Heliconia psittacorum.
  • Flow: gravity; pretreatment = 3×200\,\text{L} sedimentation/grease tanks → perforated distribution → CW → 200\,\text{L} effluent tank → infiltration trench.
  • Maintenance: pruning; desludging grease traps; flow inspection.

Water-Quality Results

ParameterInlet avg.Outlet avg.Removal %National limit (receiving water)
COD (mg/L)679.964.690.5\%150
NH_4^+ (mg/L)3.21.455.5\%–
Total P (mg/L)4.62.153.9\%–
TDS (mg/L)409.2157.661.5\%–
Settleable Solids (mg/L)3.7099.5\%1
Fats/Oils/Grease (mg/L)206.51095.2\%30
MBAS (surfactants, mg/L)53.517.267.8\%5
Fecal coliforms (MPN/100 mL)8.8\times10^42.25\times10^299.7\%1000 (reuse)
  • Meets limits for COD, solids & grease, fails MBAS, nutrients remain high ⇒ not compliant for direct discharge; suitable for on-site reuse/infiltration.

National Evidence Review (10 other Biojardineras)

  • Data scarce; BOD & COD only consistently reported; nutrients/microbiology often NR (Not Reported).
  • Reported ranges (selected):
    • BOD removal 42–94\%; COD 18–95\%.
    • Cases of negative nutrient removal (release) where maintenance poor.
  • Main technical issues: clogging, lack of maintenance, absent monitoring protocols.

General Technical Insights

  • HSSFCW show reliable removal of organics/solids/microbes; low P removal typical worldwide.
  • Optimal area in warm climates can be < 0.5\,\text{m}^2/ ext{PE}.
  • Success depends on: regular desludging of pre-tanks, solid waste control in kitchens, vegetation management.

Socio-Economic Findings

Current Wastewater Management Context

  • Multilevel, fragmented governance: AyA national utility covers potable water; 28 municipalities run own systems; rural ASADAS manage supplies.
  • Overlap of MINAE (environment) & Ministry of Health (sanitation) on wastewater control.
  • Centralized sewer expansion financially infeasible; municipalities’ tariffs cover only water supply & stormwater.
  • Household septic tanks prevail (≈76\% population); average desludging cost \$140–270; to save money, residents divert greywater to gutters (100 % households in RWL, evidenced 2018–19).
  • Discharge to gutter violates General Health Law #5395 but rarely enforced.
  • Identified necessities & constraints:
    • Need efficient greywater treatment; space scarcity; low investment capacity; desire for clear individual responsibility; acceptance of fully decentralized solutions.

Public Acceptance Survey (154 households)

  • 77\% like idea of greywater irrigating plants after treatment.
  • Plant preference: 88\% choose ornamental Heliconia over reeds.
  • Willingness-to-pay (WTP): mean \$7.4 per month for treatment service.
  • Silo mentality: residents cautious about drastic landscape/infrastructure changes yet welcome small green retrofits.

Economic Viability

  • Reported capital costs vary widely: \$188\,/m^2 (rural) – \$2500 (single household in GAM).
  • Payback time estimated \approx 3–28 years (via avoided septic maintenance), context-dependent.
  • O&M negligible (gravity flow, pruning); example fee \$0.25 month for a 4-household system.
  • Hidden co-benefits: increased property value, recreation, biodiversity, pollution reduction, green jobs.

Political-Regulatory Findings

  • Costa Rica’s Reglamento de Vertido y Reuso No. 33601-MINAE-S sets effluent limits (no distinction grey vs. black water).
  • Decree 39887-S-MINAE requires approval of treatment systems; municipalities issue building permits.
  • Focus-group synthesis:
    • Private-property Biojardineras:
    • Entities: homeowner, municipality (permit).
    • Conditions: min. 1 m from boundaries; effluent must be reused on-site or infiltrated; easier compliance ⇒ “High” feasibility within 4–10 yrs.
    • Public-space (semi-decentralized) Biojardineras:
    • Additional entities: National Parliament, MINAE, MS, AyA, MOPT, CFIA.
    • Multiple laws (Health Law #5395, Law 7600 accessibility, Road & drainage regs).
    • Complex multi-level approvals ⇒ “Low” feasibility.
  • Effluent options per regs: discharge to sewer (not available), discharge to natural water (needs permit & full compliance), or on-site reuse (Type 1; only microbial standards: fecal coliform <1000\,\text{MPN}/100 mL & nematodes).

Discussion & Integration

  • Technical performance promising but MBAS & nutrients remain challenge; more local monitoring & maintenance guidelines needed.
  • Social readiness high; citizens willing to pay modest fee; perceive aesthetic & environmental benefits.
  • Governance fragmentation & financial void are primary barriers; current legal environment only favors fully private, on-site reuse/infiltration installations.
  • Upscaling requires:
    • Adaptive or collaborative governance models engaging public, private, NGO & community actors.
    • Innovative financing (subsidies, green credits, micro-loans, cooperative funds).
    • Capacity building for maintenance & monitoring; development of national design standards.
    • Integration of NbS co-benefit metrics into policy decision-making.

Conclusions & Recommendations

  • NbS Biojardineras technically effective for organics & pathogens; suitable for decentralized on-site greywater management in Costa Rica.
  • Regulatory path of least resistance: install on private land with effluent reuse/infiltration.
  • Key next steps:
    1. Draft national guidelines for design, operation & monitoring of small HSSFCW.
    2. Pilot financing schemes linked to WTP (≈\$7/mo) & incorporate co-benefit valuations.
    3. Establish collaborative governance frameworks for installations serving clusters or public areas.
    4. Expand long-term data collection on nutrient & surfactant removal; experiment with media/plant tweaks (e.g., iron-rich substrates, polyculture).
    5. Strengthen enforcement against illegal greywater discharge while offering NbS alternatives.

Numerical & Formula Highlights

  • Removal formula applied: \text{RE} = \frac{C{in} - C{out}}{C_{in}} \times 100.
  • Prototype dimensions 1.8\,\text{m} \times 3.3\,\text{m} \times 1\,\text{m}; volume \approx 5.94\,\text{m}^3.
  • Greywater MBAS outlet 17.2\,\text{mg L}^{-1} > 5 limit ⇒ >240\% above standard.

Key Terms & Definitions

  • Greywater: domestic wastewater excluding toilet flush (blackwater).
  • Nature-Based Solution (NbS): cost-effective intervention inspired & supported by nature delivering environmental, social & economic benefits.
  • Constructed Wetland (CW) & Biojardinera: engineered wetland using substrates & plants for wastewater treatment.
  • Horizontal Subsurface Flow Constructed Wetland (HSSFCW): water flows horizontally through porous medium beneath surface.
  • Leapfrogging: skipping conventional centralized infrastructure toward innovative sustainable systems.

Case-Specific Examples & Scenarios

  • Monteverde 4-household Biojardinera: 94\% BOD removal, community-managed, \$0.25 monthly fee.
  • Guanacaste rural systems show performance variability due to maintenance lapses and negative nutrient removal.
  • Hotel Diuwak system treated both black & grey water; only 60\% COD removal owing to overloading.

Ethical, Philosophical & Practical Implications

  • Equity: decentralized NbS can empower low-income households where centralized services lag.
  • Sustainability: supports SDG 6.3 by improving water quality & enabling reuse.
  • Biodiversity: Heliconia & native reeds offer habitat & pollination services.
  • Public health: reduction of pathogen discharge mitigates disease risk.