  Stochastic gravitational waves (GWs) consist of a primordial component from
early Universe processes and an astrophysical component from compact binary
mergers. To detect the primordial stochastic GW background (SGWB), the
astrophysical foregrounds must be reduced to high precision, which is
achievable for third-generation (3G) ground based GW detectors. Previous
studies have shown that the foreground from individually detectable merger
events can be reduced with fractional residual energy density below $10^{-3}$,
and the residual foreground from subthreshold binary neutron stars (BNSs) will
be the bottleneck if not be well cleaned. In this work, we propose that the
foreground energy density of subthreshold BNSs $\Omega_{\rm sub}$ can be
estimated via a population based approach from the individually detectable BNSs
utilizing the isotropic orbital orientations of all BNSs, i.e., uniform
distribution in $\cos\iota$, where $\iota$ is the BNS inclination angle with
respect to the line of sight. Using this approach, we find $\Omega_{\rm sub}$
can be measured with percent-level uncertainty, assuming $O(10^5)$ individually
detected BNSs in our simulations. As a result, the sensitivity to the
primordial SGWB will be limited by the detector noise and the total observation
time, instead of the astrophysical foregrounds from compact binaries.
